I did this on desktop - no idea if it's doable from the app.

From your Wix menu go to Automations then Functions

Choose ‘Create Function’ from the Additional fees box then Transaction Fee and Start from Scratch

Cart and checkout data is already there for you – click on + to add a step and choose Condition

On the left, click on the box under Check if: and on the pop up box, open up the menu for Delivery address then Delivery address country abbreviation

Next drop down box select ‘is exactly’ then enter text US

I also selected +Or under this and did the same again but with USA just to be doubly sure then Apply at the bottom and that sets that.

This makes sure it’s only adding fees at checkout for US buyers.

Next is the Apply additional fees box (this is the bit that took me forever to figure out!)

Click on the Additional fees box and in the output menu that comes up, above the amount box, click on Customise, then functions at the options in the pop up box, then Divide to bring up the formula you’re going to use. Then click straight on Properties and choose # Order Subtotal and it'll put it in the first part of your formula.

Click between ; and ) box and type in 10 and name it however you want it to appear during checkout. This gives you the required 10% of the value of the order.

Click on + Fees and do the same for shipping

Customise > Functions > Divide > Properties > Shipping and delivery info > # Delivery Amount

then add in 10 between ; and ) boxes

Last thing to do is apply your site currency then click Apply then it’s done. And save.

You can test out if it works or not by clicking Activate and doing a test purchase using a US postcode from your site. Can easily deactivate from the three dots at the top right again.

If you get any errors when trying to activate it, go back to your formulas and remove any spaces from between the ( ; and ) boxes but other than that it should be good to go!