Multilple SO with diffrent Items against the same Opportunity

Hi All,

We have a requirement to charge the customer against his purchase. Each time he returns and add something new to his cart, it has to update the existing quote(opportunity). The problem here is, suppose first time he purchased an item say 'A' and made a deposit against that item. If next time he purchased another item say 'B' the quote has to be updated with both the items with merged price (due amount + new amount). So we decided to create a primary opportunity for each customer and track his/her transaction under the primary opportunity by creating multiple SO against