So I use Printful for print-on-demand items, and got frustrated setting up shipping once I realized that Shopify adds the default package weight to *all* shipping profiles, which makes no sense for shipping profiles from a print-on-demand service when you ship some of the items from your location. I sell lightweight items from my studio and have to have the package weight included in the shipping carrier calculation.
When I set up my custom shipping profile for t-shirts, I was trying to work out average weights for different t-shirt sizes, and then set up manual shipping rates for different weight ranges. It wasn't precise (mostly due to the default package weight for local shipments being added to the weight for print-on-demand items). But is kinda worked.
However, it occurred to me that the proper solution was much easier, and I thought I'd share:
Just set up a profile with a flat shipping fee for items that ship together (t-shirts, for example), then simply add the "additional item" shipping fee to the t-shirt prices themselves. So if your shirts are $25, just make the price $26.25 and advertise that "all shirts ship for the same low shipping fee".
Yeah, I know it makes the items more expensive, but in most countries it's about $1.25. Low enough to not be a deal-breaker.
And for prints, it's only $0.40 so even easier to add to the product cost.
Doing it this way makes everything MUCH easier to keep track of, and set up.
For coffee mugs, it doesn't work as well since the shipping cost is quite high for additional items. But for those items for which it works, this is a much easier solution that wasn't obvious to me when I was first setting this all up.