Shopify shipping label for large or heavy items

[ThingsMadeNew]

I have some large items that I would like to offer shipping on.  One of them would be 42"x23"x23" and 40 pounds.  I ran a test case on this, getting a quote directly from the UPS website and then creating a shipping label in Shopify.  The UPS site adds a charge (I don't know how much) for Oversized Package, and gives me a quote of $287.69.  It is under their dimensional weight and size limits, so it should ship.  My test case in Shopify gives me a shipping cost of $148.10 and a button to buy the shipping label (which I am not going to click...).  It also gives me a warning "UPS®: The package dimensions and/or weight exceeds the restrictions for the available mail services. Choose another package and try again."  I don't know why I get that warning given that it does fall within UPS limits and I can schedule shipment through their website.

If I were to purchase the label through Shopify, will it include the surcharge for oversize package, or does it ignore that and the shipping charge I pay will be too low?  If it is too low, can I still purchase the label and pay the difference when I drop off at UPS?  If a package really is outside of what UPS will accept, will Shopify prevent me from buying the label, even though the button to buy label seems to be active?  (again, I'm not going to click it to find out...)

There are significant shipping charges involved and I don't want to offer the item and get screwed because the shipping label doesn't cover all the cost or UPS won't accept the package.

Thanks for any help you can offer.

[BradleysShop]

In my experience if Shopify will let you buy it, UPS will ship it. But if your weights or measurements are inaccurate they will charge an adjustment fee (mine was $75). Otherwise the price is as stated. 

also, if you want to test the label and buy it and need to change it later, you can void the label and they refund the money for that specific label back to your account.