You're not going to find a provider that guarantees your patent will be accepted (after all, this is the government we're ultimately dealing with here). However, many will provide a refund within the first 30 or 60 days of using the service, though your patent process won't be even close to complete within that timeframe. The best indicator of a reliable application service isn't a satisfaction guarantee - instead, look through the provider's site to see case studies of customer patents that were granted after using the service.