So... the envelopes I'd ordered from Amazon turned out to be too small (inner dimensions much smaller than advertised).
I've raided Rymans and have enough envelopes to dispatch the majority of the orders tomorrow, including all of the orders with more than 1 shirt, and all of the international orders.
I've focused on doing the oldest orders first... all of these will have been sent an email, and it will all be in the post first thing tomorrow morning.
After that, there are 27 orders left to send, and I need to empty out another Rymans to send these. I will go hunting tomorrow for one.