I would ask for your specialist’s NPI number. (Not every specialist has one, so keep that in mind.) Barring that, I believe you can use the NPI number of the Costco pharmacy. After that, I would just type up a document with the codes, and attach it to a copy of your receipt. BCBS Federal is pretty awesome with this stuff.
If you don’t have BCBS Federal, but instead have another plan from BCBS, be aware that they often require the NPI of an audiologist and will not reimburse for services provided by a specialist who is not an audiologist. Often, there just has to be an audiologist in the local area employed by the same place. For example, I live in Oklahoma City. In order for BCBS to reimburse for any aids I fit, I have to use the NPI of my company’s audiologist who is in Tulsa. To make sure everything is on the up and up, I will forward the audiogram and my recommendation to the audiologist for review and approval prior to the fitting.