Try adding the actual physical dimensions (length x width x height) of the envelope into the appropriate boxes on the shipping form, and that shipping option should show up.
For some reason, eBay made this necessary some time ago.
As noted in the other post, eBay might require dimensions; I recommend that you try 12x9x1. Also, be sure that you scroll all the way to the very bottom of the list of shipping options.
When package dimensions entered on the listing form 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 Priority Flat Rate Envelopes: 12x9x1
USPS Priority Small Flat Rate Box: 8x5x1
USPS Priority Med Flat Rate Box: 14x12x3 or 11x8x6
USPS Priority Large Flat Rate Box: 12x12x6
eBay Standard Envelope: max weight 3 oz / dim 11x6x1 (actual thickness of physical envelope cannot exceed 1/4" and it must be flexible)
USPS First Class Letter (untracked): max weight 3.5 oz / dim 11.5x6x1 (actual thickness of physical letter cannot exceed 1/4")
USPS First Class Large Envelope/Flat (untracked): max weight 13oz / dim 15x12x1 (actual thickness of physical envelope cannot exceed 3/4" and it must be flexible & of uniform thickness)
