I'm in a similar position to the one you were in.

Listing an item with 'Seller pays' postage and Evri de-selected. eBay is recommending Small Parcel up to 1kg. My item exceeds the max dimensions for Small Parcel by 2cm on one dimension (other two are within), but also weighs nearly 3kg.

I posted for advice - was it safe to go with eBay's recommendation or should I switch up to Medium Parcel which my item's size and weight is fully within? - and most members who replied said the package would be accepted with the recommended label as the item's size and weight are within the maximum limits for Royal Mail delivery, even if they exceed the limits for Small Parcel.

I spoke to the manager at my main PO about this earlier today. She echoed what those posters advised me (which is also stated on eBay's SD help page), but said that, if was her, she would select Medium Parcel.

I'm still yo-yo'ing back and forth between the two options, which is ludicrous, really, as the difference is only a few quid, and my item will be priced at at least £400. ["at at" - who do I think I am, General Veers?!]

I'm curious more than anything to know if things would go smoothly if I did go with eBay's recommended band, but it would be a potentially problematic test probably better left for a much cheaper item. Despite the reassurances from other members (and eBay itself), under the current circumstances I'm not particularly inclined to trust eBay to follow its own protocols.

Incidentally, the PO manager told me that Simple Delivery (she wasn't familiar with the name but did know about the 'Marketplace Seller' text on the labels) was, and continued to be, an absolute headache, and that the communication about it had been awful, although she was probably talking about the Horizon system comms (and possibly comms between The Post Office and Royal Mail). The confusion is understandable, as accepting what would, under 'normal' (non-SD) circumstances, be an underpaid label must be a completely foreign way of working for them.