"Our freight terms are FOB shipping point, and payment terms are 1/10, net 30. This vendor has recently fulfilled several of our purchase orders, shipping materials from overseas plants. The time from shipment to receipt was approximately 8 weeks. The vendor invoiced the date the materials shipped from the overseas plants."
As far as common practice, according to TAPN's Disbursement Survey (2008), three-fourth's of participants calculate disbursement timing from the invoice date; 12.6 percent go by invoice receipt date; only 3.7 percent go by product or service receipt date, and 8.9 percent by "other" -- where timing is most often according to specific contract agreement, but also according to authorization.
In general, it would depend on the agreement, but if the contract or agreement does not specify, then you must work it out with the vendor. You will have to get the appropriate parties together to agree on what date to go by.