Balsa wood, that's a good idea! At least there's no risk of it stretching out of shape into a bigger hole, which could happen if the cardboard was a bit thin.

I asked my PO for one and they said they don't keep spares, they only have one for each counter. If one breaks, they share one between them until a new one arrives. That's why I made my own.

I find it really useful when I'm creating new listings. I send large letter items as 'free' post and charge postage for parcel items. If something looks like it's boarderline, I stick it in a padded bag and drop it through the slot and then insert the postage cost if needed (or not). Saves that OMG feeling after someone buys it and you realise that you've advertised with free postage and find it's too thick and has to go parcel rate.....which costs more than the buyer paid for the item!