I haven't done co-ops, but I sell on Etsy and Ebay, so I ship a lot.
If you have boxes that are the right size, you might as well use them (provided they are sturdy and still in good condition).
Do you have a postal scale or a kitchen scale? If so, you can package them (mark them so you know which is which), weigh them, and print out shipping online. Paypal has a decent multi-shipping function (useful if people are paying via Paypal), though there are other sites. Online shipping is usually cheaper than going to the post office. You'll need packing tape to secure the edges of the label if using paper instead of label sheets, so that it won't rip off in the mail.