Don't think I can add anything but to hammer home the facts.
Buy regular forever stamps for the first ounce. Don't care what they cost bought 200 a few years ago just getting to the end of them now.
Buy Additional ounce forever stamps for both the second ounce and to have them not put it through the the sorting machine.
Buy two stampers one that says "Non-machinable" and one that says "Do Not Bend". Stamp both on the front and back.
Over 18 cards your going to need to send as a package. Use PayPal shipping. Current rate on an ounch package is 4.something. With Paypal its a little over $3. DONT go to the Post Office! It is amazing the incompetence you will find there.
The only time I go to the Post Office is if I'm shipping internationally because it requires a custom form. If I'm sending to Canada and it not ovewr an ounce they have International stamps that work just fine.
Sorters will only flag stuff like that if they have someone looking over there shoulder and its king of obvious. go over on postage to remove hesitation.
Happy trading