The required payment solution needs:
1. Shopping cart where visitor can register and buy different quantities of credits for varying amounts.
2. Buy buttons (credits) which will allow access a chat room for a specified amount of time
3. Users can "tip" other chat room users using their credits. Therefore the tipped users account will have those credits applied to it and deducted from the tippers obviously.
Thats about it.
PLEASE COME BACK WITH SOLUTION SUGGESTIONS AND PRICING. IF YOU PROPOSE TO USE A THIRD PARTY CART/SOLUTION PLEASE HIGHLIGHT AND EXPLAIN YOUR METHODOLOGY AND CUSTOMIZATION PROPOSED. IF YOU PROPOSE TO USE OPEN SOURCE SOLUTION EXPLAIN YOUR EXPERIENCE WITH IT INCLUDING EXAMPLES OF WORK YOU HAVE DONE.