Single postcards or small groups of postcards can be sent in a #10 business envelope with a stiffener.
If you print Do Not Bend on the envelope, then the envelope goes as a parcel which will cost much more. A stiffener like 110 pound cover stock folded twice works great.
Have a postal scale handy.
Print the postal rates for first class then tack it up where you can see it easily.
Use priority mail envelopes when you have 100 or more postcards to send. The flat rate is $4.95 regardless of where the item is going in the USA, and regardless of what the weight it. Pretty much what ever you can fit into the envelope is OK. Postage can be printed from the PayPal page.
Be sure to have a few of each type flat rate envelope and boxes on hand.
If you have several hundred postcards to send to the same address, mixtures and the like, then you want to investigate the flat rate boxes offered by the post office. For $10.80 you can really get quite a bit of material in a medium size flat rate box. There are three sizes. Keep a few of each size on hand.
You can print the postage from the PayPal page which saves going to the post office.
You will need to speak with your route letter carrier to let them know you will be sending items from your home from time to time.
Try to mail every paid order out the next day after payment is received. That is goal of an efficient business. Getting behind is painful.
Click on these hyperlinks to visit the main pages of this OldPostcardsForSale.com website:
Find items of interest to buy in this book rack from Amazon.com:
Old Postcards For Sale 2006-2011