Sounds Like you need to upgrade to 7.4 and use the new Service Catalog features.
Is this the only way possible to do this, do you have any information on the service catalogue features in 7.4?
Multi-select list boxes would also sound as though they would fit the bill but again those are for 7.4 only I think.
In 7.3 there is a control called a paged control which allows you to define a query to put on a window, search for items in the query and then multiple select. It then copies the items selected into a text field. You can also remove items
You could also consider adding a customer action to the process which allows people to select one item at a time, but then press the action again and select another. Personally I'd go for 7.4 as there are heaps of other reasons to upgrade too, but I do understand it's not always possible to do an upgrade at the drop of a hat.