If I understand correctly, then yes, you can set a promotion rule that *increases* the selling price, instead of decreasing it based on the criteria you define. The other approach is to set a required product relation, whereby buying one product automatically forces to buy another.

Thanks for replying. I don't think that I was very clear on what I'm trying to accomplish. So let me take another shot at it.
I want to use the module as a Bill Pay for a client who is in the garbage business. They have several base priced services, coupled with add-on services. So instead of listing each services, what I would like to do is simply enable the customer to type in what they want to pay. Maybe they'll pay last months bill, or maybe pay for the whole year.

Hi,

Currently we don't support that feature but it would be something we would look into doing in the near future.

Thanks,