There is a current ebay glitch with the mobile phone app that shows listings with calculated post or freight - as free post in ebay, (in the postage details page of the app I am pretty sure it shows that freight is applicable- but I doubt many buyers look at this section on the phone app) - so IMO I don't think your buyer is telling you fibbers.

If it was me, I would ring ebay and ask them to send an explanatory email to the buyer = they probably won't, even though it is their error, but doesn't hurt to ask.

In any case, I would also messge the buyer and explain that you have now been informed of the app glitch, and ask him does he want to cancel the sale and take it from there.

In relation to keeping your post costs down, depending on the type of goods you sell, you may wish to consider purchasing a pre-paid 5kg satchel they cost $16.70 and are relatively large (510 x 435mm) - you can pop well packaged boxed goods in, if extra packaging is needed to keep breakables safe, and avoid paying cubing and distance $$$

If you do use any pre-paid products, to ensure you get paypal seller protection and have relevant proof of address and lodgment, my advice is to complete an AP lodgement form, cross out express post platitium and write in parcel parcel satchel in that spot and tick box - complete all addressee details and get your post office to stamp it on lodgement of goods.  Lodgement receipts are available from post office or available on-line easily found via google.  Attach the tracking sticker to the lodgement form, and you have all the paypal required proof.