Execution Usage Limit! Argh!

I'm trying to run what I think is a fairly simple script to do the following when an Invoice is submitted. Occurs for each line item on the Invoice:
- get serial number
- search for this serial number in Item Receipt records
- return the value of the field 'poastext' (purchase order number)
- put this value into a custom field on the line item in the invoice

This is so when we do reports on sales, we can reference the sale of an individual serialized item right back to the originating PO. Some vendors require this when we report sales back to them.