QUESTION: payment package

Hi,

What I Would like is  that:

A user could post an ad as a 'vendor' only if he buys a package for that only ad.
that is to say : each time he posts an ad he would have to pay to have the 'vendor' status on it. Otherwise his ad will be normal satus (no vendor) :

Example:
-If he wants to be 'vendor' for 3 ads he would have to pay 3 times.
-if he wants to be 'vendor' for 5 ads and  'simple user' for 2 ads then he would have to pay 5 times .

How would you do that?

Membership Duration :
Credits :
Max :
Upload Images :
Upload Videos :
Upload Docs :
Bold ad title :
Premium Ad :
Vendor URL/logo :

Thanks for your help