diff --git a/lib/cash-in-tx.js b/lib/cash-in-tx.js
index 6eee0b1f..07319764 100644
--- a/lib/cash-in-tx.js
+++ b/lib/cash-in-tx.js
@@ -12,7 +12,7 @@ module.exports = {post, monitorPending}
 const PENDING_INTERVAL = '1 day'
 const MAX_PENDING = 10
 
-function post (tx, pi) {
+function atomic (machineTx, pi) {
   const TransactionMode = pgp.txMode.TransactionMode
   const isolationLevel = pgp.txMode.isolationLevel
   const tmSRD = new TransactionMode({tiLevel: isolationLevel.serializable})
@@ -21,17 +21,17 @@ function post (tx, pi) {
     const sql = 'select * from cash_in_txs where id=$1'
     const sql2 = 'select * from bills where cash_in_txs_id=$1'
 
-    return t.oneOrNone(sql, [tx.id])
+    return t.oneOrNone(sql, [machineTx.id])
     .then(row => {
-      return t.any(sql2, [tx.id])
+      return t.any(sql2, [machineTx.id])
       .then(billRows => {
-        const oldTx = toObj(row)
+        const dbTx = toObj(row)
 
-        return preProcess(oldTx, tx, pi)
-        .then(preProcessedTx => upsert(oldTx, preProcessedTx))
+        return preProcess(dbTx, machineTx, pi)
+        .then(preProcessedTx => upsert(dbTx, preProcessedTx))
         .then(vector => {
-          return insertNewBills(billRows, tx)
-          .then(newBills => _.concat(vector, [newBills]))
+          return insertNewBills(billRows, machineTx)
+          .then(_.constant(_.concat(vector, machineTx.bills)))
         })
       })
     })
@@ -39,14 +39,23 @@ function post (tx, pi) {
 
   transaction.txMode = tmSRD
 
-  return db.tx(transaction)
+  return transaction
+}
+
+function post (machineTx, pi) {
+  const pp = require('./pp')
+  pp('DEBUG98')(machineTx)
+
+  return db.tx(atomic(machineTx, pi))
   .then(txVector => {
-    const [oldTx,, newBills] = txVector
-    const oldBills = oldTx ? oldTx.bills : []
+    const [, updatedTx] = txVector
 
     return postProcess(txVector, pi)
-    .then(changes => update(oldTx, changes))
-    .then(tx => _.merge({bills: _.concat(oldBills, newBills)}, tx))
+    .then(_.tap(pp('DEBUG99')))
+    .then(changes => update(updatedTx, changes))
+    .then(_.tap(pp('DEBUG100')))
+    .then(tx => _.set('bills', machineTx.bills, tx))
+    .then(_.tap(pp('DEBUG101')))
   })
 }
 
@@ -146,19 +155,19 @@ function convertBigNumFields (obj) {
   return _.mapKeys(convertKey, mapValuesWithKey(convert, obj))
 }
 
-function pullNewBills (billRows, tx) {
-  if (_.isEmpty(tx.bills)) return []
+function pullNewBills (billRows, machineTx) {
+  if (_.isEmpty(machineTx.bills)) return []
 
   const toBill = _.mapKeys(_.camelCase)
   const bills = _.map(toBill, billRows)
 
-  return _.differenceBy(_.get('id'), tx.bills, bills)
+  return _.differenceBy(_.get('id'), machineTx.bills, bills)
 }
 
 const massage = _.flow(_.omit(['direction', 'bills']), convertBigNumFields, _.mapKeys(_.snakeCase))
 
-function insertNewBills (billRows, tx) {
-  const bills = pullNewBills(billRows, tx)
+function insertNewBills (billRows, machineTx) {
+  const bills = pullNewBills(billRows, machineTx)
   if (_.isEmpty(bills)) return Promise.resolve([])
 
   const dbBills = _.map(massage, bills)
@@ -166,17 +175,16 @@ function insertNewBills (billRows, tx) {
   const sql = pgp.helpers.insert(dbBills, columns, 'bills')
 
   return db.none(sql)
-  .then(() => bills)
 }
 
-function upsert (oldTx, newTx) {
-  if (!oldTx) {
-    return insert(newTx)
-    .then(tx => [oldTx, tx])
+function upsert (dbTx, preProcessedTx) {
+  if (!dbTx) {
+    return insert(preProcessedTx)
+    .then(tx => [dbTx, tx])
   }
 
-  return update(oldTx, diff(oldTx, newTx))
-  .then(tx => [oldTx, tx])
+  return update(dbTx, diff(dbTx, preProcessedTx))
+  .then(tx => [dbTx, tx])
 }
 
 function insert (tx) {
@@ -198,8 +206,7 @@ function update (tx, changes) {
   .then(toObj)
 }
 
-function registerTrades (pi, txVector) {
-  const newBills = _.last(txVector)
+function registerTrades (pi, newBills) {
   _.forEach(bill => pi.buy(bill), newBills)
 }
 
@@ -218,17 +225,18 @@ function logAction (rec, tx) {
   .then(_.constant(rec))
 }
 
+function isClearToSend (oldTx, newTx) {
+  return newTx.send &&
+    (!oldTx || (!oldTx.sendPending && !oldTx.sendConfirmed))
+}
+
 function postProcess (txVector, pi) {
-  const [oldTx, newTx] = txVector
+  const [dbTx, updatedTx, newBills] = txVector
 
-  registerTrades(pi, txVector)
+  registerTrades(pi, newBills)
 
-  const isClearToSend = newTx.send &&
-  !oldTx.sendPending &&
-  !oldTx.sendConfirmed
-
-  if (isClearToSend) {
-    return pi.sendCoins(newTx)
+  if (isClearToSend(dbTx, updatedTx)) {
+    return pi.sendCoins(updatedTx)
     .then(txHash => ({
       txHash,
       sendConfirmed: true,
@@ -237,23 +245,46 @@ function postProcess (txVector, pi) {
       error: null,
       errorCode: null
     }))
-    .catch(err => ({
-      sendTime: 'now()^',
-      error: err.message,
-      errorCode: err.name,
-      sendPending: false
-    }))
-    .then(r => logAction(r, newTx))
+    .catch(err => {
+      // Important: We don't know what kind of error this is
+      // so not safe to assume that funds weren't sent.
+      // Therefore, don't set sendPending to false except for
+      // errors (like InsufficientFundsError) that are guaranteed
+      // not to send.
+      const sendPending = err.name !== 'InsufficientFundsError'
+
+      return {
+        sendTime: 'now()^',
+        error: err.message,
+        errorCode: err.name,
+        sendPending
+      }
+    })
+    .then(r => logAction(r, updatedTx))
   }
 
   return Promise.resolve({})
 }
 
-function preProcess (oldTx, newTx, pi) {
+function preProcess (dbTx, machineTx, pi) {
+  // Note: The way this works is if we're clear to send,
+  // we mark the transaction as sendPending.
+  //
+  // If another process is trying to also mark this as sendPending
+  // that means that it saw the tx as sendPending=false.
+  // But if that's true, then it must be serialized before this
+  // (otherwise it would see sendPending=true), and therefore we can't
+  // be seeing sendPending=false (a pre-condition of clearToSend()).
+  // Therefore, one of the conflicting transactions will error,
+  // which is what we want.
   return new Promise(resolve => {
-    if (!oldTx) return resolve(newTx)
-    if (newTx.send && !oldTx.send) return resolve(_.set('sendPending', true, newTx))
-    return resolve(newTx)
+    if (!dbTx) return resolve(machineTx)
+
+    if (isClearToSend(dbTx, machineTx)) {
+      return resolve(_.set('sendPending', true, machineTx))
+    }
+
+    return resolve(machineTx)
   })
 }
 
diff --git a/lib/pp.js b/lib/pp.js
index 8aa7b76c..e82754fd 100644
--- a/lib/pp.js
+++ b/lib/pp.js
@@ -1 +1,6 @@
-module.exports = o => console.log(require('util').inspect(o, {depth: null, colors: true}))
+module.exports = function (label) {
+  return function (o) {
+    console.log(label)
+    console.log(require('util').inspect(o, {depth: null, colors: true}))
+  }
+}