I put their answer here because it can be useful for other people :

“We operate on a net 30 basis, so subscriptions renew 30 days from the date of your initial subscription. If you want to unsubscribe from an API, simply go to the Pricing page of the API and select Unsubscribe from the applicable plan. The date of your new subscription will then be reset as the 30 day trigger for future renewals.”

For specific billing questions please send an email to mashape support…

support@mashape.com

Will my $50 replace automatically the $10 ?
Or should I remove the $10 (before ? after ?)

yes