Pricing for item be %age of prev item?

Hi

We sell products with service contracts that are prices based as a percentage of the product - essentially annual maintenance.

At the moment I'm using markups to calculate it, which works well for the purposes of quotes.  I'm concerned that when we eventually switch on the accounting side of things that I won't have a way to bill maintenance in the subsequent years.

I also sometimes need to apply a line-item discount to these items (and have the two numbers print on the quote).  In quotes, I can have both a markup and a discount.  In opportunity items, it gives me the error "Posting and non-posting discounts/markups are not allowed on the same transaction".  Given that I don't have accounting turned on, I cannot find a way to determine what is "posting" and what is "non-posting".