This response may sound a little harsh, but I don't mean to be harsh. It's a frustrating situation for all these days and a bummer when it's hard to buy things we want.
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Along with what others are saying, it probably also has to do with what they can get right now. They order MUSA clothing from a small shop in New Hampshire (I think). That shop has multiple customers, and some production capacity to balance, and who knows what the raw materials and labor situation there is. QBP can't even keep hubs in stock. How is Riv going to get any priority from suppliers when their order quantities are fractions of most companies?
Combine that with increased demand - for lots of good reasons in Rivs case - their products and brand are becoming more and more well known and desirable - and you end up with out of stock stuff. It's a bummer, but I don't think Riv can do too much about it - otherwise they'd be selling more bikes and employing more people.
And when Riv does stock up on stuff (like they did for the retail store), it ends up sitting on the shelves and putting them in a tough spot. It's a damned if you do, damned if you don't situation, so IMO choosing "don't" has a lot of supporting arguments.
Will's restock emails are an interesting peak into how often they get orders from different suppliers, and stuff just doesn't seem to be rolling in frequently or in large quantities.
Paul in AR