OK, so I went on the Post Office Web Site, and the one she had talked about is the one that MTS just mentioned:
Priority Mail Padded Flat Rate Envelope $4.95 12-1/2" x 9-1/2"
It's the same size and rate as the standard paper one that comes at the post office, only it accommodates bulky items.
The next size that might fit for fabric:
Priority Mail Small Flat Rate Box $5.20 8-5/8" x 5-3/8" x 1-5/8"
Only a 25c more. Does anyone know if 5 yards of fabric fits in this box?