I send old postcards to the U.S. which is comparable to sports cards
I always use Bubble Envelopes (Size 0) with double cardboard backings
have never heard of someone saying they will arrive faster using just plain envelopes
my delivery times for the last few years has been between 2 days to just over 2 weeks depending on where they are sent in the U.S.
Sending to specific locations these times have varied within that time frame
think it has more to do with customs clearance and sorting time then using a certain type of envelope
Personally I would never send my cards in a Plain Envelope to much of a chance of them being damaged with the envelope being so flimsy
never had any damage using bubble envelopes
weavers