If you are having trouble completing a purchase because you have your in-game steam overlay disabled as pictured below, follow these steps to re-enable the overlay in order to complete your purchase.
1. Navigate to the Steam tab in the top left of your primary steam window, click settings, then navigate to the In-Game section as pictured below, then make sure "Enable the Steam Overlay while in-game" is checked.
2. Right click on Icons: Combat Arena, go to Properties, and make sure "Enable the Steam Overlay while in-game" is checked.
You should now be able to complete your purchase!