Thank you for sending your pictures to eXtension.

Considering how quickly the plants began to decline after planting, I'm leaning toward transplant shock, or some other post-planting stress. Unless the entire garden center was turning brown like this, I would not expect mites to be able to find your five plants so quickly, and in such numbers as to cause this.

This publication, although somewhat dated, contains some general information on transplant shock: https://www.extension.purdue.edu/extmedia/BP/BP-31.html

I am assuming, of course, that you removed the containers from around the root ball before planting (please don't laugh, I actually found that once during a landscape visit!).  When the container comes off is a great time to examine the roots.  Healthy roots should be a creamy white color; you should be able to see small roots and root hairs.  Unhealthy roots will be brown or black, slimy, and there will be few small roots visible.  Sometimes, end-of-the-season bargains are sold cheaply because the roots have started dying back, due to heat, over-watering, or under-watering.

Although we had a very wet spring, for most of the state, the rains pretty much dried up in June.  Newly transplanted stock should be watered once or twice a week, being given 1/2 gallon of water per square foot of bed or transplant hole.  Were you providing this much water?  I've seen many cases where people have actually drowned their plants by overwatering.  These are the folks who water every day, or turn on the hose and let it run for a spell under the plant.

Did you ever actually see any mites?  Did the garden center tell you how to look for them?  If they didn't, they were being negligent or lazy.  True professionals would not sell you a chemical without at least checking to see if a pest is there first.

To check for mites: get a sheet of white paper, and hold it underneath one of the sick branches (not a dead branch...there won't be any mites left on a dead branch).  Give the branch a couple of smacks with your hand or a rolled up newspaper.  If there are mites, they will drop to the sheet of paper.  After a moment, they'll begin to slowly crawl around the sheet.  A magnifying glass will show you the small bodies with 8 legs.  Dirt and debris may also fall, but it won't have legs.

It is my opinion, based on the pictures and your description, that these plants died soon after being planted, or they were dying before you even got home with them.  I would check with your garden center to see what their return policy is.  If they refuse to allow you to return the plants, you might want to shop at a different garden center in the future.

Please let me know if you have any other questions.

Spider mites are very tiny.  Slightly larger than a period at the end of this sentence.  That's why we test them on a sheet of white paper: to make them stand out.  Those are regular garden spiders in your picture, and they are eating other flying insects.  They are good guys, so you can leave them alone.

Again, I think we are dealing with a failure to survive the transplanting, as discussed above.