if my experience of Alpkit is typical, they’ll order three of each size, sell the standard sizes immediately then keep spamming a “sale” on the XXXS and XXXL sizes for the following year or so.
My experience of Sonder is that their stock levels are usually pretty good and “sales” beyond their pre-used demo bikes are pretty limited. Obviously if they’re going to sell out of any sizes then it’s likely to be the more common Medium/Large varieties so will typically have the less common sizes if there is a sale going on.
I don’t see Sonder as doing anything different in this respect to any of the mainstream manufacturers and nothing like the Planet X approach.