For my customers they have to add their UPS/FedEx account and PO# before getting to the cart. Our developer told us that was due to Shopify not allowing customization in the cart. Customers of course ignore this even though we have a pop-up before checkout reminding them you can’t add your account later.
It looks like there’s an app that may help. Does anyone have experience with this. Do I need a developer to add this in?
If you are trying to add custom fields to the checkout then I believe you can only do this with a Shopify Plus plan (if you are on that plan then yes, you can achieve it with an app like the one you linked, and without a developer).
Otherwise, I’m afraid you’ll be limited to collecting the information at the cart only (before customers reach checkout), which it sounds like you’ve already tried.
Its hard to say without looking at your store but I imagine it can be tightened up a bit more to require customers to enter the information before providing them with a checkout link, to avoid them skipping it. But you might still find yourself in situations where they’ve skipped straight to the checkout one way or another.