If you have a promo code, you enter it at checkout to get a discount on your purchase. It works for both plans and top-up packs.
If the code is valid, the discount is applied and the new total is shown. Then finish paying as usual.
You can apply the code first and then switch currency (USD / EUR / RUB). The discount percentage sticks, so you don't need to re-enter the code after changing currency.
Apply checks the code against our database and tells you why it was rejected:
One more answer can appear later, at the payment step rather than at Apply: "This promo code is no longer valid. Remove it and try again." That means the code was one-time or limited and has been fully used up by the time you paid. There's nothing to fix on your side. Remove it and pay at the regular price, or use a different code.
Every currency has a minimum charge. If a discount would push the total below that minimum, the payment provider raises it back up to the minimum. The checkout explains this when it happens.