Stopping buyer to pay, until I send an invoice (to apply new US tariffs)

[wwmeteorites-25]

Hello, 

with the application of new US tariffs on import from the UK (from where I am selling), I will apply a 10% cost on my sales, to US eBay customers only (as 10% of the value will be collected from me by Royal Mail on the package content value, at the moment of shipping, on behalf of US customs).

The above is for context, not needing an answer (this is not a question), here is what I am puzzled by and need an answer for :

How to stop buyers to pay for item(s) before I did send them a manual invoice, where I add the 10% ?

In other words, I only want my buyers to be able to pay ONCE I did send them an invoice, but not before. Is their a tool/function to do that.

Thank you

[*vyolla*]

At this stage there isn't a workaround here, we're all waiting to see if eBay will come up with something, though I suspect it will only be for items sent via their Global Shipping programme.