1. Is there a billing system (AWBS, ClientExec, Ubersmith, Modernbill, WhoisCart, Whmcs, ...) having "access levels" functionality? For example, user Admin1 can do everyhing, but can not cancel invoices; Admin2 can view reports only; Admin3 can do all, but can not view reports; etc.
2. Is there one supporting 2checkout.com recurring payments?
3. Are there convertion/migration utilities allowing to change a billing system, e.g. move from ModerBill to ClientExec or vice versa?
1. I know ModernBill does offer this (4 different access levels I believe, but you would have to verify this). As for the others, I cannot comment.
2. 2CO recurring payments are very difficult to track as they do not send an IPN (to my knowledge) for each payment, as other competitors do, such as PayPal. I am currently using MB4, and I know that it doesn't offer the feature to use recurring payments with 2CO.
3. There are conversion tools available, and they usually come with the billing software. I believe WHMCS offers a few converters if you are choosing them.
Just to add to adakist's number one, WHMCS would best fit your needs, however I extremely recommend modernbill for it's functions and power, as I do believe WHMCS as a few security tweaks that are needed to be worked out.
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I know that ModernBill and Lpanel support the different access levels. I don't believe any of them have full integration of 2co recurring payments. I believe when ModernBill V5 inclues the 2checkout module (I think V5.2) it will include 2checkout recurring.