Shipoff: Shipping Offer

This app allows me to combine shipping when I sell some products using print on demand, and also shipping some products from home inventory. Otherwise shopify was charging shipping twice because it does not recognize that one purchase is being made from two locations, meeting my free shipping threshold for customers.
It does cost a small amount per per month, but pretty cheap. Also, I was able to get ahold of customer support, but actually ended up not needing them!