Google says include VAT but Discount coupon comes out wrong

Hi all

I welcome any advice. The prices on our website used to add VAT at the checkout. This was fine until Google Shopping told us that now we had to include VAT on our prices or our Product Listings would be banned from Google Shopping.

So we included VAT in our product prices, but now we have a separate problem: When someone signs up for our free guide, they get a 10% off coupon. Sadly, the coupon takes 10% off the including VAT price.

So I bought chair for £85 + VAT, it shows up as

Sub-total £102.00 (inc vat)
free shipping so £0.00
Discount coupon : - £10.20
UK VAT £15.30
Total £91.80

So in a nutshell, it should be minusing £8.50 not £10.20

This is the basic discount coupon that comes with ZenCart. Nothing has been added on. I welcome any help of how I can work around this.

Thanks in advance

Put it this way: How can I keep Google happy by including VAT on the product prices but enable the proper calculation at checkout?

Has anyone had any similar issues?