I had exact same issue on my private account ebay changed my long term listing to SD and even the items marked as large parcels previously using parcelforce at £12 they changed to evri max 4kgs £3.25 for both. I had no idea and when 2 items sold at was 125cm each and evri would not accept it, speaking to ebay rep in india somewhere he closed the live chat on me. I then had to arrange my own postage at my loss. The whole system is possibly one of the worse ideas ebay have ever imposed and is ironic they call it Simple as its completely the opposite. There needs to be more realistic option and especially a fix button where ebay have got it wrong

use whichever workaround above that suits your listing format:

I think I may have solved this. I too had an issue with the length of a package, mine was 70 x 50 x 50cm but the max was Medium Parcel. When listing an item, Under Delivery Details you'll see 'Package Size (Optional)' .I simply filled in the weight and dimensions and then underneath that it said 'Simple Delivery is unavailable for this item', and a Primary Service option popped up under that, where you can input your custom delivery options. Hope that helped!