there is a classified MOD but it's for free using the user login from SNITZ, what you can do is add a new field to the USER table say: "advertisement" from type bit (1/0) and in the classPlaceAd.asp add a script that performs a query requesting this field... if "1" then show the Post Button, if "0" disabled all fields (you can use jquery to do this in just 3 lines of code) and hide the Post button adding a message (a div) saying that, to post they need to buy a membership...
build a new page with access to paypal (for example) and when retrieving the information that the payment was successfully, UPDATE the user filed value to "1" and voilá!
Updated: instead of messing with the USER table, create two new tables with this fields:

[ADS_PAYMENTS] USER_ID
PAYMENT_DATE
PAYMENT_VALUE
DURATION (in days or months)

[ADS_ALLOWEDMEMBERS] USER_ID
ACCESS_UNTIL (PAYMENT_DATE + DURATION)

so you can save a log of payments and present a message (or send email) to users one month before the credit expires... for example.<

Originally posted by Carefree
Or you could use the existing subscription mod and if they've subscribed (and it's current), let them post advertisements.

For this kind of things, you should always use a new table to hold logs like payment date, amount, full message from paypal, paypal transfer id, etc... so you can get this later on or provide an invoice to the user.
lets imagine that a user cant pay... should always be saved the attempt and error messages (in an error or NotSuccessful table to not mess up with the Successful payments, dont you agree?<

but he wanted to that be subject of an online payment ... you should save the payment information!<

guys, we are in the 21th century and you use Excel ???
we have databases to keep everything in one place, we have invoice software, and so many new things, you are kind'a late on this matter I can see.
you do as you want, I do like I do, and to tell you something, I've been invoicing my programming work for more than 10 years now without any money spend in book keeping personal! At least I know what I am talking about ... humpff... excel!!!<

this is the same as, "why do we need to use Source Control?? I always did my backups and can always see latest versions of my code"

the answer is the same, after you do things in the right way, we will never go back, and you will end up saying "Why didn't I use this before! so headaches so worries... for nothing"

I have my share part of 'Excel', but there are MUCH better, there are tools (DB) to keep our stuff in order... why am I bordering.
as said, before, you all do how you love more, I know, from my end what I am talking about, I've been there and look back I hated! I just wanted to give the best choice to the fellow.<