What is the Date of Service for pharmacy items ordered by mail?

Is there an authoritative rule establishing the Date of Service for pharmacy items ordered by mail? My insurance company wants to use the order date and I want them to use the purchase or shipment date. It’s a difference of several hundred dollars.

I’m in Oregon, if that matters, and the insurance company is Regence BCBS.

Longer version:

In January 2022, I ordered some OTC COVID-19 tests from Amazon, knowing I would be reimbursed by insurance. At that time we were constantly learning about exposures, and going through the tests at a steady clip. Local supply of tests was spotty at best, but Amazon seemed to have a consistent supply. I set up a standing order for tests via Amazon’s Subscribe-and-Save program. My insurance said they’d cover 8 tests per person per month. The monthly order I set up was for 6 tests per person.

After 3 months, tests had become more available locally, so I cancelled the standing order. I submitted a claim for reimbursement with receipts. My insurance covered some of it, but there was some confusion over dates that I’ve been unable to resolve.

On their receipts, Amazon unhelpfully lists the “order date” as the date I first set up the standing order. The receipts do include the actual purchase/billing date (which is also the shipping date), but less prominently. Naturally, the “order date” is what my insurance is insisting on using for the Date of Service on these claims.

I’ve appealed the claim decision several times. It’s a difference of several hundred dollars. It was initially tricky to figure out exactly what was going on due to health insurance complexity (and seemingly willful obfuscation). On appeal, they’ve shifted around the distribution of tests per person, but they’re sticking to that Date of Service.

It seems natural and logical to me that the Date of Service for a mail-order COVID-19 test should be the purchase date or shipment date, as opposed to the date a recurring order was initially submitted. The insurance company has been pretty firm that no, it’s the date the order was placed. I asked for some written policy or guideline to back this up. The representative couldn’t find one. She said they’d send the claims to be adjusted yet again and I could go from there. I am not optimistic about that.

So, is there any authoritative rule or guideline establishing the Date of Service for pharmacy items ordered by mail?