The only way I have found to go around this poorly contructed issue, is add the billable to the invoice, copy the description, choose my product/service and then paste the description back in there along with the original cost. The markup's on the other hand you can set to a specific income account and will not have to touch those (if you utilize markups that is). 

Funny thing is, if you are entering a bill and need to change the expense account.....it will let you WITHOUT changing the description. Curious on how they can manage to keep the description and the amount of the item in a bill while changing the expense account but they can't figure out how to keep it from deleting everything in an invoice when selecting a product/service item to add after attaching the billable to the invoice. Go figure......