Simple and easy! You choose your prefered currency, then choose vouchers with predetermined values from between 10 and 300. Then go to your favourite merchant and redeem your voucher using a unique voucher pin number that you will receive when purchasing the voucher. Good news is that if you do not redeem the entire amount, the balance will remain valid on your account for up to 14 days and you will be able to redeem the balance using the same unique voucher pin. If for some reason you are not able to redeem the balance of your voucher within the specified 14 days, then the balance will be refunded to you.
If you prefer, then you can custom create vouchers to pay for any specific product at the merchants site. As an example you may need to buy a product for $43 USD. You can therefore choose your prefered currency and then purchase a voucher for that exact value. The same principle applies with the unique voucher pin. If for some reason you get to the merchant site and you receive discount on your purchase or decide to purchase a product with a lower value, then the same principle applies with the balance which will remain valid for a 14 days or a refund if balance is not used in the 14 day period.