This is only happening when selecting the merchandise plans, correct? Any combination of your other plans does not cause a problem? Also purchasing via merchandise ONLY functions fine as well, correct? Sounds like an issue still between recurring and single payment plans. Merchandise can only be single payments and when a user attempts to make a recurring purchase with a single payment only purchase perhaps is the issue. Please check the durations of your plans and ensure they are correct. Some recurring plans depending on the payment gateway require a minimum duration such as perhaps 5 days, etc...