Having been caught out after sending things I'd sold on eBay a few times, it's all about the dimensions, and under 2kg is cheaper. For post office, small parcel, it needs to be a box something like a maximum of 15cm deep ( x bigger than A4 on the other sides) . It's worth googling and measuring as otherwise it's a fair bit more expensive.
Let me see if I can find a good tree up here, if not I'll take some off you. It's been a good year for beech, but the conkers are poorer and not yet found a good tree for acorns.