Wordpress niche site for art needs to filter/exclude some buyers...

Hi. I already have Stripe and Paypal integrated into my Wordpress site. But what I am trying to 'fix' is that all my prices include S&H to continental North American locations, but NOT to any location outside N. America. I need some way to stop customers who live outside this continent from buying and paying for purchases. I need them to first inquire about the cost of shipping to their location so that I can adjust the pricing for them before they buy. I already tell them to do this, but sometimes they don't. The last two purchases were from UK users who needed to pay an extra $130. in S&H costs. The first paid it. The second asked for a refund, which cost me the same Stripe fees, even though I had to cancel the sale and refund their money. I don't want that to happen again, which is why I want to prevent them from paying until they agree to the extra costs. So I don't want to stop them from buying entirely, just want them to contact me and agree to shipping cost first. Then I can adjust the price for them and they can buy it, after which I will change the price back to what it was before, for the next customer. Can u help?