How do you send an invoice with updated postal charges, to combine postage etc?

[ImaLannister]

Does anyone know please how to do the above?
I recently sold something on eBay and I tried to send an updated invoice in the drop down section on the item, but every time I tried to amend the p&p charges then pressed recalculate the amount stayed the same. It also says enter (-) or (+) and I tried every way.
I do it on the eBay actual website, you know www. not the app. Is this my problem?
I ended up having to send the buyer £3.95 straight from my PayPal to theirs. But eBay would have charged me fees on that £3.95, so I would rather that didn't happen.
Any help would be appreciated.
Thanks.

I sold 2 items to someone last November, and this is from memory lol.
I'm using the website, not mobile or the app.
I go to my selling page, where there is a list of items sold. To get here hover over the My Ebay then click "selling" on the drop down menu.
The two items will be grouped together awaiting payment. I think I ticked the boxes to generate a combined invoice.

This is where it gets fuzzy, because I've only done it once, so I'm handing over to ebay help page.

see ocsnext.ebay.co.uk/ocs/sr?&query=289
Sending a combined invoice to a buyer

Thank you! I will try that. They don't make it easy do they 🤔

I have done it in the past but memory also gone a bit sketchy. I do remember the first ever time I added the amount on instead of deducting it so double check prior to sending invoices. Very embarrassing. These days no one takes any notice of my sentence on listing saying to request invoice from me to combine postage which must be done prior to payment. So I always end up refunding a partial payment instead Hense why I have forgotten how to do it all.