We have tried a few things.
Drop shipping would be best but the minimum to be able to do that with split orders is $90/month for a shipping upgrade (again I would maybe break even on a good month, maybe). I did an experiment with drop shipping hats, the only instructions on the hat were “please do not combine this with hardware orders or they will be refunded” …yup, most of them were mixed orders and I lost money on each hat sold, so I just upped the price.
Direct link to Dan’s paypal product, could work as at least a button from the parts page. Dan has just implemented a size option and we are giving it a test period (probably proven itself by now). Not as slick as a link from the shop though.
The only other option is shipping them in bulk to me…probably the next step.