Charging customer an agreed amount for postage

Customer calls or emails and a shipping fee is agreed.
How to enter it on the order form?
Any reason not to give the customer a coupon name to enter eg shipping10 and then then when the coupon is applied, it adds $10 to the bill, because the coupon, shipping10, has been precoded with a negative dollar amount?
Any better way to do it?

You can't have negative amounts for coupons.

Usually, what people do is this:
You remove the payment view from the checkout and activate the option "Payment of orders not paid immediately".
People place orders on your website.
You add the shipping fees to their orders and you notify them by changing the status of their order.
They will receive an email where you can put a link to their order listing where they can click on the "pay now" button to pay for their order.

For 90% of our customers,who do not pay for shipping, your suggestion would be more cumbersome for the majority?

Why cannot you use a negative amount in the coupon definition. It seems to work and adds the required amount to the order

Oh it works ?
Well, that's something we didn't thought about when we did the system, but if it works for you then that's great.
I thought that the problem was that it wasn't working

Yes using a negative amount does appear to work but I wondered if this was a deliberate feature or inadvertent? Sounds as if it is inadvertent but will nonetheless work and not have any unexpected consequences. Fingers crossed

It was not deliberate, but if it works then that's great and I see a fee applications for that actually.