Let accounts buy more room from My subscription

My subscription shows what the account uses against what its plan and add-ons allow, one card each. Add more opens the add-on that raises that limit to pick the quantity or the tier, and offers only one on sale in the account's currency and country; when the account's plan leaves that add-on out, the button reads Change your plan instead. A prepaid balance that sells no packages of its own is topped up the same way, from the add-on that grants it.