Print on demand and custom-in shop items on the same order

[Laraneo]

Say on my shop I had t shirts listed but are produced by print on demand retailers, but I also have items that are handmade by me. Since I have my own shipping rates, package sizing, etc. How would it work when a customer purchases a print on demand shirt (from a different supplier) and a handmade item in the same order? Regarding shipping? 

@Nick