Another android dev with (much) lower volume here. Curious how you know about errors driving that 20% number. Is there a report somewhere with this detail or is it just inferred from customer emails? The only failures I ever see come through are declined credit cards or the regular cancellations from users who used the 15min refund window.
If a customer just refunds, then the order just says "Cancelled" and the details of the order say that the card was successfully authorized.
It's those people that WANT to give you their money but get stopped because of a Payment Decline or Google Cancel that are ridiculous.