Generally there is no tax on legal funds.
You are familiar (not personally I hope) with the fact that money laundering, terrorist financing, tax evasion, and other illegal financial activities happen in this world, right? And that the perpetrators of such crimes are unlikely to make truthful customs declarations?
What sort of purchase are we talking about?
In several countries — and the number is growing — it’s illegal to pay cash for large purchases, where “large” isn’t very large. Cash (and cash equivalents) are also quite dangerous to carry to/from/within many countries. Demonetization and volatile exchange rates can be financially hazardous. And you have no recourse even if everything goes well in the immediate transaction but the merchant sells you a lemon (and you weren’t trying to buy a lemon, the fruit).
So go get an ICBC Singapore Global Travel Mastercard that’ll pay you approximately a 0.5% net rebate every time you use it to make purchases overseas. Much, much, much, MUCH safer, too. If you want a higher credit limit than they’ll approve, you can place a fixed deposit with ICBC and use that to raise it. (Ask first, of course.) ICBC’s fixed deposit rates are rather good right now, especially if you open one via their e-banking.