"I had to remove 131 items I had up for sale because eBay changed the postal settings across the board - meaning for some items I'd be over or under-charging my customers. "
If you mean the 131 items were changed from whatever postage you had entered to Simple Delivery and showed different prices; then there was no need to remove them. Simple Delivery prices are different to Royal Mail and Evri normal prices. And even if you had previously priced it as a e.g. a Royal Mail 1KG medium parcel, if ebay had priced it as a Simple Delivery 1KG small parcel then the SD postage label would still be valid*.
"I was charged £5.40 for 2nd class recorded " Sounds like you bought completely new postage - there's no 2nd class recorded in Simple Delivery. If so, if the buyer paid postage they will, in theory get the SD postage cost back eventually. If you paid the SD postage, you can request it back. Also it is unlikely to track as delivered on ebay, so you will need to wait longer for payout.
You "need"** to use the Simple Delivery label. To get a Royal Mail label (AFAIK) you either need the QR code to show at the post office or to print your own label.
With Evri you also get a 6 character number and then could go to a Evri parcel place to get a label i.e. you don't need a printer, QR code, mobile phone etc. just pen and paper.
(*) If you had items which were above the maximum size/weight for the post office/royal mail (or Evri) and had previously offered a courier/rate which could handle those sizes/weights; then you would have an issue if they were switched to Simple Delivery. For any of those items, if you can't change them back to the "big/heavy items" courier yourself - I suggest you contact customer services.
(*) [Also a seller should not try and try and fool the system e.g. by in anyway implying the size and/or weight of the packed item is less than reality.]
(**) Well you don't need to use the SD label, but ebay may not like it, and you will wait longer for payouts.
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