You have the following options:
Purchase yarn while online and add the sock cranking service to your cart, or
Stash bust your stash, send your yarn that you want cranked into socks after you purchase the sock service. (Make a note at checkout that you will send the yarn to the shop)
Choose your stitch count by measuring around your foot while accounting for the need for negative ease in your finished sock.
FINGERING weight stitch counts: