What in-app or in-game purchases are offered in your product?

Players can purchase GXP, a currency that can be used in the GXP Store. The GXP store offers: skins that increase characteristics. In addition to skins, there are consumable items that will help in leveling up and fighting.

Can your product be used without making any in-app purchases?

Yes

Are in-app purchases refundable?

You can return the spent GXP to your account within 14 calendar days after purchasing the erroneous product by contacting support(http://help.gamexp.ru/
).
Refund policy: https://help.gamexp.ru/kb_article.php?ref=2037-EPSL-8244

What payment platform is used to facilitate these purchases? Do you use Epic Games’ platform or a 3rd-party (including your own) payment system?

We use the Epic Games platform