Are you sure you're listing in one of the eligible categories? If so, then you should know that eBay's wacky programmers have recently introduced a completely unnecessary hoop to jump through for sellers who want to use the Standard envelope and Priority Mail flat rate packaging: Sellers must enter specific dimensions! This is what member wastingtime101 wrote about the situation:
eBay made an unannounced change related to setting up listings with certain types of packaging. When package dimensions entered exceed eBay's programmed dimensions, eBay will not show you those shipping options when setting up your listing.
To access these services, enter the following package dimensions:
USPS Flat Rate Envelopes: 12x9x1
USPS Small Flat Rate Box: 8x5x1
USPS Med Flat Rate Box: 14x12x3
USPS Large Flat Rate Box: 12x12x6
eBay Standard Envelope: 11x6x0.25
