How to top up a local SIM without getting overcharged as a tourist?

Use the official apps (Dialog, Mobitel, etc.) for self top-up with a local payment method or international card where accepted. Many small shops add a small service fee. Ask for the exact amount you'll receive before handing over money. Supermarkets and official agent shops are usually more transparent than random street vendors. The difference is usually small, but being aware helps. Having a local friend or your guesthouse host top it up for you can also avoid any confusion.