Yes, each time a message is read the credit balance will be reduced by 1 credit
I would also need to update/flag the message as 'read'.
So the ecart can be configured to offer say 4 credit options and then credit the users credit balance with the value selected and bought.
The sort of thing i have in mind would be
5 credits = $8,
10 credits = $15
15 credits = $20
20 credits = $25