angra

Couldn't OP just take their bag to the closest combini and send it from there to the hotel?

I have another thought that is further outside the box of the original request.

If you expect that the stuff you want to buy would fit into a carryon sized rollaboard, you could bring a collapsible soft-sided bag (e.g. a duffel bag) with you, and kept in your regular bag until you reach your final stop, at which point you expand duffel bag and allocate between check and carryon as required. This is what I do when I go to places that I expect to buy souvenirs.

I've never had a problem with a normal rollaboard on a shinkansen or any train in Japan aside from a crowded commuter train.