Hi @Alma_Barlow - you can do this:
IF(
AND({We Ship}, NOT(Shipped)),
{Est Ship Cost}
)
Which translates to:
If We ship is true and Shipped is false, then return the Est Ship Cost
Note that there’s 3 other possible scenarios for the checkbox combos and with the above all of these will return null or empty, so you might want to expand the formula to return zero or other values for these cases