How to ship from Canada to US for FBA

I’m going to apologize since this is probably a stupid question.. I have a newborn and zero functioning brain cells!

I’m a Canadian seller and most of my customers are in the US. I make a product in house and did a trial run selling on Amazon US (FBA). Happy with the results, I boxed up $5000 worth of product to send. Previously I had shipped to Amazon by UPS, This time the declared value will incur some customs charges. I’d be happy to pay but UPS is only showing options to ‘bill receiver’ or ‘bill third party account’. What is the third party account? Do I need a customs broker?

What do other Canadian sellers do to send product to US? Once again, sorry for the dumb question! I’ve previously shipped orders directly to customers so this has me stumped. Any info would be appreciated.