you would need to iron that fq pretty flat to get it in a long business envelope and have it slip through the cardboard letter check slit. i send fat eighths through the mail all the time and i know i iron and steam them really good to be able to send in the long envelopes for either $.44 or $.64 depending on the weight of fabric. i will have to try a fq and see if i can get that flat enough to fit through that letter check slot in the long evelopes. otherwise the postage jumps up to $1.71 for the package rate.
unless you have a weight scale at home to weigh your letters and packages, it would be hard to use the usps.com site to calculate postage.