If program exists in market then receiving a fee it's only valid with Market's Payment processor
3.3 You may also choose to distribute Products for free. If the Product is free, you will not be charged a Transaction Fee. You may not collect future charges from users for copies of the Products that those users were initially allowed to download for free. This is not intended to prevent distribution of free trial versions of the Product with an “upsell” option to obtain the full version of the Product: Such free trials for Products are encouraged. However, if you want to collect fees after the free trial expires, you must collect all fees for the full version of the Product through the Payment Processor on the Market. In this Agreement, “free” means there are no charges or fees of any kind for use of the Product. All fees received by Developers for Products distributed via the Market must be processed by the Market’s Payment Processor.
Which basically says free apps are OK, but if you want to sell a full version of these apps you can only do it through the Android Market. When the Android Market first opened paid apps were not supported and some developers directed their suers to go directly to a 3rd party app store or their own payment portal and ran into issues with Google. I do believe you could get around this by encouraging users to go to your website and then have the paid version information available on your website.
grCor