I do the same thing -- put my baggies into the SASE, have the clerk apply the appropriate stamps with a bit extra, then put the whole thing into a larger envelope addressed to Mariposa. My SASE fits perfectly into the largest Priority Mail Tyvek envelope.
BTW, Honchey, 30 is the maximum number to swap, but if you want you can do less. This is the first swap where the number was increased to 30. Prior swaps had a maximum of 20. I think Mariposa is a glutton for punishment, that's why she upped the ante. Come join us!