Hi there @gem5, and welcome to the forum!
I provided a similar solution here Join Retool-Table with API-Query - #2 by MiguelOrtiz
The only additional thing is that you would have to format this additional custom column as boolean and in the "mapped value" section write the logic using item to returb true or false, e.g {{ item > 1 ? false : true }}. This will format your column with an empty checkbox if amount is higher than one, and with a tick (you can edit to change from tick to cross) otherwise.
Let me know if you keep struggling and do send some screenshots of what you have tried.