The cards I received are all 14 x 9 centimeters, which is a little smaller than the usual postcards. You don't need to make them larger; the ones I got are all 14 x 9.
You can order custom postcards from online stores, and they typically use paper that is 300 g or heavier. The most common types are copper-plate paper or white card stock, which is about 300-400 grams per sheet. Other textured papers are also fine; just choose the one you like.
Please note that some papers require specific pens or stamps for writing or stamping. My cards are double-sided copper foil paper (without a protective film). Both the fountain pen and ballpoint pen I bought from the stationery store can be used normally. However, after applying the protective film, only the ballpoint pen can be used; the fountain pen cannot. Regarding stamping, please ensure that the ink and stamp oil match the surface material. Since the paper has been coated with a protective film, it is not absorbent to water or oil, so I haven't made any rubber stamps yet.
You can try using the packaging boxes you already have at home. Some cosmetic boxes have a protective film, which provides readily available materials for experimentation. Also, some oil-based pens or ballpoint pens are waterproof, but they may not be resistant to alcohol (such as some blue oil-based ballpoint pens). If this is important to you, consider using different pens instead.
Generally, your QSL card will be well-preserved by the receiving station. The quality of online-customized cards is not a major issue. If you need to order in large quantities, it's more cost-effective to use an offline printing shop; typically, starting with 1,000 copies is more economical, and they seem to use 300 g copperplate paper, costing around 200 yuan for 1,000 copies? If you only need a small number, it's best to order online.
When exchanging cards, I usually send the cards to other stations via regular mail, with one envelope and two QSL cards (no more than 20 grams), along with a stamp. If I want to track the progress of the exchange, like using express delivery, I would use registered mail, which costs an additional three yuan. This means that each time I exchange a card, it will cost me one yuan. I have never used a QSL card before, and I don't usually exchange many cards.