Cezm, regarding those 'letter guidelines', note that the postal worker at the post office puts the items though the 20mm slot. Surely that's the time to flag an unsuitable item to the client?

OP,   you really need to keep up with  Ebay and AP announcements.    AP    has clamped right down on people sending parcel  articles as letters.    New sorting and scanning devices can pull an item from further shipment at any point during delivery process.   Any such items can be returned to sender, or additional charges and penalties can be applied to senders accounts.  If the  sender can not be ascertained then additional charges and penalties can be applied to the recipient or delivery refused.

So it really is up to you to meet the shipping requirements and relevant costs.

I noted that the item indicated it would be shipped by  AP small box/satchel, which is what the buyer would have been expecting in the first place.  As the item (including original packaging) would well and truly fit in a small box/satchel  (max 5kg Flat Rate)  it is laughable that you state the original packaging will not be included to reduce postage costs.   It does however tend to offer an insight into your choice to  "save" on postage.

Best of luck with your transaction