Add shipping manually at cart end

Hi everyone. I have a situation where I have customers who need a custom freight amount charged due to the size of their purchase etc. I would like to be able to give them a price to ship X product, and when they add that product to the cart, they can then manually add the quoted freight charge themselves - either directly or by way of a code / voucher / coupon etc. I have though of using the Opencart coupon feature for this but that only allows for discounts, not additional charges. Can anyone think of an effective way of doing what I want?

Well, you better post such Customer-Requests in the commercial section,
to make sure, that it will be seen by Dev's, not regularely visit the open
Forum Sections. And others will most likely not be able, or willing, to assist
on such highly Custom Mods, just for the plain fun of it (anymore).
Sorry for the bad News, it's nothing personal ...
Ernie
--
(Still, one might, just to prove me the contrary )

A discount is just a negative fee, so you should ask the developers of the extensions you've seen for this, and see if they can give you a small code edit to allow for fees. It should work for adding the shipping cost like you want.

Alternately, you could just add a $1 product to the store, and customers can add the quantity of that item to add the shipping cost. For example, they'd add 50 of that product for a $50 shipping cost. (You could also do that with a $0.01 product if you needed exact shipping costs.)