Is a module for the required checkout available?

Hi, I am new to open cart and am having a problem with the default checkout.I am hoping
that someone has a module or can adapt a module to help, or
recommend something or someone who can help with my problem.

I have an ebay shop which suffers because the checkout is no good for the
products I sell. If a customer buys one product, or multiples of it then
checkout is straight forward. ie item 1 is charged at full postal rate,
then each additional item is added at a fixed rate. - This works.
The problem arrives when two or more different items are ordered because
we have to ask customers to wait for us to send an invoice, - or we have
to issue a partial paypal rebate if they pay when they order. Quite often the
customer cancels because the postage appears too high (each separate item
being charged at full postage rate instead of additional postage rate).

I have been putting together an Open Cart shop, and what I am looking for is a
module that:- A. puts an individual postage cost per listing plus cost per each
additional item (as with ebay).
* PLUS *
B. prompts the customer to request a postage total if two or more separate
items (either singles or multiples) are ordered, * OR * a facility that sorts
out the highest individual postage price and then adds the rest of the ordered
items at their individual fixed additional rates.
Mr.Sticky