Start with a minimum spend that is not more than you can afford to lose. Then when the upvote is shown to come through, you can do a little bigger spend next time. After a few spends with upvotes received, you will have greater confidence that upvote will continue coming through, and if it stops you stop paying for more but will have already profited more than the lost upvote. Always verify upvote comes through before next spend.
Also on a new service you can check their history on Steemd to verify that upvotes are being sent after payment.

Always do your own diligence; verify that result is in line with the stated service and that your expectations are in line with reality.