Packing slip checker
A small system that checks three documents that should agree and usually do not: the purchase order, the packing slip, and the physical count. The engineering is straightforward; the design problem is that the only moment a discrepancy can be recorded is the moment nobody has time. Seven posts on the same system, one diagram at a time, with a cost breakdown and an engineering reference at the end.
- 01
A packing slip checker on AWS for a few dollars a month
The whole system on one page — three documents, one comparison, one moment — and why the receiving doorway is the hardest interface in the design.
- 02
How the three documents get compared
How the order and slip are matched before the delivery arrives, how the physical check list is chosen, and why accepting the rest on trust has to be an explicit decision.
- 03
How a discrepancy gets recorded at the door
The constraints of the receiving doorway, what the interface must and must not ask for, why the delivery note is annotated before signing, and what the driver gets.
- 04
What happens to over and under shipments
How a shortage is escalated the same day, what to do with goods you did not order, why rejections stay with a person, and how the invoice ends up matching.
- 05
How discrepancies become a pattern
How individual discrepancies aggregate into supplier, product and unit-of-measure findings, what the recurring cases usually turn out to be, and what to report.
- 06
What the packing slip checker costs
About $407 a month. One Bedrock read per delivery is the only line that grows; the queue, the table, the mail and the storage are rounding errors. Plus the three ways the bill could surprise you.
- 07
Engineering reference: the packing slip checker architecture
Same system, drawn purely for engineers. Service names, region, Lambda inventory, IAM scopes, the schemas and the exact model id.
Frequently asked questions
- What is a packing slip checker?
- A small serverless system that compares a purchase order, the supplier’s packing slip and the physical count at receipt, and records any discrepancy at the moment it can still be proved.
- Why not just check the packing slip against the order?
- Because the packing slip is a claim about what was sent. It agreeing with the order tells you nothing about what is in the boxes, which is the only thing that matters.
- What happens when something is short?
- It is recorded with evidence at receipt, the delivery note is annotated before signing, and the supplier is told the same day. The post on this covers why the order matters.
- Does it automatically raise claims or reject deliveries?
- No. It records what was found and tells the right people. A rejection is a commercial decision and a claim is an assertion of fact.
- What does it cost to run?
- A couple of dollars a month. See part six.