Per an email I received today, University Products is offering free continental-US shipping this week (27-31 October 2020) on mat board. They offer two product types under their Lineco brand - "conservation" board (100% alpha-cellulose) and the more expensive "museum" board (100% cotton rag). I've been using Lineco white museum board for several years.

I just placed an order and found I needed to enter the discount code towards the end of the process, on the payment page - it wasn't recognized when I entered it on the cart page. Just make sure to enter and apply it before placing the order.

The discount code is FREESHIPMB. I have no relationship with UP other than as a satisfied customer.