What sort of software is this? Mass-market software selling for $5 needs a completely different solution from special-purpose software selling for $5000.
If the software is over $150 or so, it's probably better to setup your own payment system down the line.
If you are trying to lock down a shareware program, armadillo is the best.
The ASP is worth joining to get the best info if they're doing a lot of this: http://www.asp-shareware.org/
I would recommend you avoid SWREG and other Digital River properties. Our experience with DR was very unpleasant and a quick Google search on "swreg scam" and a review of this summary: http://successfulsoftware.net/2007/07/04/swreg-customers-bew... should tell you why.
Golden%Braeburn strikes me as something that would be great, but I'd really only want to pay for it once.
e-junkie + paypal and/or GoogleCheckout is one non DR option: http://successfulsoftware.net/2007/07/23/cost-effective-soft...
Plimus, Avangate and Fastspring are others.
Also, a smart programmer can provide a better solution that anything digital river can offer.