You’ll want to go with USPS First Class Packages for shipping. I hope your labels fit the boxes.
I started out creating labels on usps.com when volume was really low, like a few orders a month. I quickly stepped up to the free option for stamps.com but because of the rule changes with first class packages, I can no longer print stamps and separate label and put them on the small boxes I had.
I now have to print large shipping labels that require 6x9" padded envelopes to accommodate the size. I ended up going with Endicia for printing postage (primarily because it works in Wine) but that has a large enough monthly cost ($15) that it wasn’t worth it until I reached a certain volume of sales. If I add up the discount savings versus the monthly fee, I’m almost certainly in the hole. Margin on product makes up for it somewhat.
Don’t forget that the Tindie fees are calculated on product price + shipping price.