Amazon Go

I'm assuming your Amazon Go account is tied to a credit card. In which case, it's not Amazon's problem. They got their money - you now owe your credit card company.

But that card could be maxed out.

And if your credit card company declines the charges when Amazon attempts to bill them? Then they're out inventory and don't have any compensation. Then it becomes their problem.

Maybe they ping the card with a $1 charge every so often to ensure it's at least not cancelled. It could be maxed out in which case they'll likely just give you a negative balance and a time period to pay it. If you don't they can just sell it collections and potentially ban you from the store (it looks like you need to scan your phone to get into the store).

In some cases it is still possible to go over a card limit through the way transaction processing works. For instance, when you go to a restaurant and pay in the US they will run the card for the amount of the bill (which may not decline because you have enough $), but you might add a tip on the receipt. The tip will be added to the final charge and taken from your account, potentially making you over the limit. This is usually never stopped but might cause you fees and banking headaches. They may implement a similar procedure like gas stations usually do, by pre-authorizing a fixed dollar amount such as $50 or $100 and sending through the real value to collect after.

They could do like gas stations and charge a hold just whenever you enter the store then finalize the transaction whenever you leave. That'd solve the problem and if it was going to fail you'd just never be able to enter the store and you'd just see a warning message about your CC not being valid.