Your projects may have certain selection items where the clients can pick anything they want, but you still have a basic selection that you have priced into the contract. In this case, you want to give the client the ability to go with your basic option, but also to upgrade to whatever they want.
First, create a selection item with a single predefined option, which describes your basic selection. Then, either enter a price of $0 or a price that matches the allowance amount. Either way, choosing that option does not increase or decrease the contract price.
As the project progresses, your clients may go to a vendor to find upgrades or may add comments to the selection item asking you to price various options. When this happens, simply update the selection item by adding those additional options to the predefined list, along with their prices. When the client finally goes to make a choice, they are able to choose from your basic selection and all of the other options they requested.