You will be expected to pay for things in the local currency 99.5% of the time (THB in Thailand, MYR in Malaysia). Other related fun facts:
- You should keep about 20-33% in USD initially on arrival to the first country. Exchange the rest into local currency. No big deal to exchange 100% at once, but then you lose with multiple exchanges when you hop to the next country and then when you return home.
- Also, USD 100,50 bills get better rate than 20,10,5,1.
- No USD or MYR coins are exchanged in Thailand (only paper money is exchanged; exceptions may exist at certain Thai/Malay borders). but i think Malay exchanges accept coins without any issues.
- Malaysia money exchanges might raise the rates of non-major
currencies on the weekends/holidays. ATMs are not affected.
- Thai/Malay train border usually have an exchanger on hand. Rates are acceptable. Get enough there to last you at least a day or two in the other country, especially on the weekends.
Make sure to exchange back to USD (or other major currency) before you go home. Otherwise when you get home, you will likely have a terrible exchange rate for THB,MYR.
The charge will likely be in the local currency (THB, MYR) and your bank auto-calculates it based on the visa/MC rate.
If the vendor gives you option to charge in USD (which is rare for in-person purchases), make sure the total price is about right compared to the local currency equivalent.
But there may also be separate "foreign currency transaction fees" (either separately or also added to the conversion. So check with the card issuer for conversion charges from the base account currency)
foreign transaction fees.
Similar for cash withdrawals.
FYI, majority of Thai ATMs will charge 220 baht for non-Thai card withdrawals; it is added as part of the total withdrawal with an alert. Malaysia ATMs seem to be "fee-free". So try to pull money out when you reach Malaysia.
If you are lucky, you have a card with <=1% total foreign transaction fee. But I've seen people with foreign transaction fee 3-4% of transaction + $3-5 USD fee per transaction. (Yes, banking cartel gets away with it). And the fee structure might be different for cash withdrawals vs credit charge purchases.
I would avoid CCs except for high ticket items from established mall/business or if it is an online merchant you trust (and know exactly what the charge covers). It's up to you really, but especially Thailand is very cash-based. And God forbid the order is wrong or needs to be modified......have fun.