I have created a recurring charge with capped amount of $49.99. I checked the Recurring Charge API and its showing capped amount correclty ($49.44) in the result.
However, when I try to create a usage charge above $49.99 say, $486.72, the usage charge is getting created without giving any error regarding the capped amount exceeded and when I pull up the Usage Charge API result I can see the balance remaining as some negative number.
I check the balance_remaining before trying to create the usage charge. Perhaps that's why I haven't encountered that error. I don't think it is safe to rely on the Shopify API to catch that situation (which I think it should check and return error) I have an uneasy feeling about possible bugs (edge cases?) in RecurringApplicationCharge & UsageCharge so I strongly suggest checking what you reasonably can, and logging responses in case of disputes.