So I have ordered from the Marketplace twice.    Which is these is the standard transaction?

The first time went smoothly.

The second, not so much.    I ordered end of April-they notified me a week or so later it might be slow due to pandemic issues.  I would have really liked a bit more detail on what that means, but fine.  Today (a full month later!) they've cancelled half the order because it is "no longer available" (you couldn't figure out that in less than  4 weeks?!).   Yes, they sent a refund, but the shipping part was only the "additional shipping" added for each book, which means the effective per-book shipping cost  (total shipping/# of books) has gone up a lot on the remaining order.  When I spotted a book I wanted in the marketplace I spent a LONG time looking for more books to add to lower the per-book shipping, and now that was all a waste of time, and  the remaining books' effective prices are now on par with Amazon (and Amazon doesn't charge me credits to get to that price).  On top of all of this--I still don't have any books!  

So which experience should I expect if I were to make a third marketplace order? Which I probably won't for a while anyway because finding books is such a pain, but do I write it off completely or is the current mess just due to pandemic and bad luck?

I'm sorry part of your order was cancelled. I've not had that happen on my orders. My last order did take longer than usual. I can understand the COVID 19 slowing things down as less people are working so they can social distance so the warehouse likely has a fraction of the workers.

I had a similar cancellations with Walgreens and Target. They cancelled the part of the orders I wanted and added other items to get free shipping. It was frustrating. I'm assuming the computer programs said the items were in stock but when they went to fulfill the orders that wasn't the case. 

I've had two Amazon orders cancelled too and both items were available when I ordered then the orders were delayed and about a month later both cancelled. So I am thinking there was a supply chain issue.

I hope you have better luck if you do order again.

I ordered once from Marketplace about 3 years ago and it took 4 weeks for the book. So I haven't done it again - but it's true there are some major supply chain issues and that could well be the reason.

The delay isn't the main problem.  It's just one more thing on top of the rest.  I wish they were more clear about how long to expect, but I am reasonably patient if I'm getting a discount, and recognize that life is chaotic right now.   Honestly, if they refunded the actual per-book shipping price I'd be sad (because I wanted the books) but not upset, because I'd still paying what I expected for the books that were coming.    It's the fact that I'm paying extra due to their screw up that irritates me.

That would irritate me too!  You spend all that time to make it a good deal for you, and *poof* - the deal is gone.  Can you cancel the remaining part of the order?

I have no advice really, just wanted to say that PBS is basically a 'middle man' for these transactions, so a lot can happen in the time between placing the order and the actual bookseller pulling the book(s) from stock and mailing.  I've only ordered once or twice through the Market and that was soon after it was opened, so several years ago... I don't recall any problems, but I decided I wasn't in any hurry for ANY books, so have pretty much quit buying anything other than ebooks...