Here's the way the card system works.
First of all, postage is not cheap anymore. Back in the 50s and 60s it was about 3 to 5 cents to send a post card. Things have changed.
You need cards, envelopes stamps and a shitload of ones.
Stateside, Puerto Rico and US possessions rule of thumb: Your card and an SASE. If it is a Special event I generally send a buck to help them out.
DX: Your card, SAE and money for postage. This is fair. Round UP when sending dollars. $1.01= $2.
Postage generally runs $1-3 but there are exceptions. IIRC Iran is about $13 or more.
Paper DX QSL adds up fast. My rule of thumb is that a DX crd runs about $5 when it is all over and done with. Truth is that's a bit inflated, though. You add the card (a quarter) the stamp to get it there ($1.20) three bucks postage and when you add the envelopes it hits close to a five dollar bill.
Pro tip: Go to a Hallmark store and ask them for envelopes and they'll give you a shitload fo free! Get 2 different sized envelopes so they nest.
Pro tip: Some overseas QSL cards do NOT fit into the small personal letter sized envelopes we use here in the states. go with an envelope at least 4x6.
Pro tip: It often costs the DX station to have US dollars changes so don't scrimp.
Pro tip: Think for a second. You are NOT going to get 100% return. (I run 90 to 95% on returns) Plan on it. You are mailing to places with questionable mail service. The mailman in places delivers the mail with a goat and gets paid by what he can steal. Jamaica has a real problem. I got Jamaica confirmed by Canadians on vacation after a couple tries with Jamaicans.
Pro tip. Check the QRZ page of the DX operator and do as he says. IF HE WILL ACCEPT PAYPAL DO THAT! It may be a buck more because PayPal takes their bite but it insures he gets his money. It's worth it because sending cash is risky.
Pro tip: Go to the operators QRZ page and email him first. He may have a change of address etc that isn't on his QRZ page.
Pro tip: Every 6 months write your QSL bureau and send them a bunch of SASEs and ask them to send you what they have for you. You have to ask. Generally they have something for you.
Pro tip: Don't email the DX station for 4 months and ask them where your QSL card is. Things often take time. I had an Italian ham tell me it took her 45 days to get my card. Mail service in the other part of the planet runs between better than ours to outright dismal.
I like paper. It's expensive but I like it.
LoTW is the way to go if you want to work for awards.
Pro tip: Bookmark this.