After a customer has clicked the "Proceed to Checkout" link in their quote to load their cart and discount, they CAN always still ADD additional items.
When using the Coupon Method to discount a quote:
What they can't/shouldn't do..
Remove items from the cart:
When a discount has been provided.
Our discounts require all items from the quote be in the cart for them to receive the discount you provided.
If items from the quote are removed, so to will the discount.
Items can be added and/or qtys increased
What if I know they're choosing between the items I am quoting, but I don't want them to be able to arbitrarily change qtys.
Send each item on a new quote.
We make this super easy ;-)
Hit the copy button at the top of the quote > change the item, > and send!
The customer can now choose the quote they want to purchase, and the link on that quote will load the items(s) and discount properly
Using Exact Cart Pricing Method
Customers can change qtys, or remove any items, and the price of the item will stay fixed at what is was set at on the quote.
In general, the fixed cart pricing is our recommended default method. The coupon method is best used for when you don't want to allow the customer to make any changes to the qty or list of quoted items and not lose the discount quoted.