How can I correctly set up different free shipping thresholds for bulky items in my store?

Hello all, I am getting closer and closer to finishing my new store before I go live from another provider. With this said, we do have a strict requirement. We sell small little items as well as large bulky items where we get to pay a lot for shipping. So… I want most small items to qualify for free shipping with orders of $100 or more, no problem, this is done. Where the issue comes in is I want to increase that threshold to $150 or even $200 for the bulky items, so when a customer buys Bulky Item #1 they have to spend $200 or more to get the free shipping. I got this to work well creating another shipping profile and adding the bulky items to it. Where the issue comes in is how Shopify calculates the shipping rates for a mixed cart.

Examples:

Shopping cart contains:

2 x regular items, then Shopify renders a fairly accurate shipping rate

Shopping cart contains:

2 x bulky items, then Shopify renders a fairly accurate shipping rate

BUT! When the shopping cart has a mix of regular and bulky items (both shipping profiles), then the shipping rates grow exponentially out of control as if Shopify is combining both calculations together, which I get it should, but this doesn’t work because even when the customer reaches the free shipping threshold, the customer will still be presented with these astronomical shipping rates for UPS, but yet cheap rates for USPS and customers can chose their preferred method and leave us making a lot of extra money or losing money.

What am I doing wrong or missing from this shipping profile system?

Thank you!

Bert

I guess nobody has experienced this?