In UK we have a list of items for "Hospital Bag".
To prepare for the arrival of the baby we do buy few things so that can be worn when the baby is born.
Native Americans would enjoy Italian traditions, then. It's considered to be terribly bad luck or tempting fate to buy things for the baby before birth. Like taking too much for granted. It's assumed to jinx the fate of the child.
Anyway, if you do stock up totally, there will be nothing left for them to buy you, and they will feel not only like you didn't honor tradition but they got cut out of the fun.
Make a list of all the stuff that you could use but don't have to have the moment the baby is born, or maybe is a little more luxurious than you would spend on yourself. (For example, a specialty stroller rather than the standard kind, or a certain kind of chair for the baby's room that otherwise you would do without.) You will have a lot of time after the baby is born before you need certain things, like locks for the kitchen drawers and a highchair or bouncy chair. All of that stuff can therefore be on the list you give your in-laws. If they don't get it for you, you still would have lots of time to get it later.
And in the meantime, you can quietly stock up on items newborns totally have to have. Just keep it low key. You're bothering them by not heeding their tradition.