Putting aside how ridiculously bad a gap this is for Squarespace, I used weight-specific pricing as a workaround.
I have three similar products (playing cards) and then another one which is uncut playing card sheets and considerably more expensive to ship. (https://www.cartesiancards.com/shop)
So I assigned very low weights the regular cards, and made the last one weigh lots more.
You have to be careful with people adding multiple products if you want them to stay in the same price bracket. Hopefully I've avoided this by exaggerating the weights like I have.
One more new product that's different to the others and I'll probably have to make the move to Shopify.
