I've used Autoslash a few times, but not that specific feature. I've never had an issue.
I participate in a frequent traveler forum (like DisBoards but for people who travel a lot...mostly on business). On the car rental forum, there's an Autoslash employee that answers car rental questions. She or he also occasionally answers general travel questions on other part of the forum. That employee always gives great advice, particularly when it comes to rental car issues.
If the rest of their employees are as competent as the one posting on that forum, I'd guess you won't have any issue. Of course, that's just a guess. Hopefully someone who has done the pay-now option will come along and provide an answer based on experience.