I've been a fruitcake recently...
Just been hit with a few ebay packlink package oversized payments, rookie mistake I know, but just got complacent thinking I didnt need to measure boxes anymore, doh.
If you use the Ebay uk packlink services you should definitely check parcel size, as it seems in the past they didnt care if you were a few cm's over, but now they are more vigilant now, got charged £4.16 (+VAT) for each parcel :(
For UK people, if sending via UPS (best way for parcels valued over £50 and bigger than royal main small parcel size), the way UPS calculate the max parcel dimension (max size limit is 180cm for parcels up to 10kg), take the parcel 2 smallest dimensions and times by 2, then add 80, this total value needs to be under 180cm for weight class up to 5kg. The 10 to 15kg weight class has a bigger max dimension size (235cm).
I had been using the boxes that lego ship the big sets in (UCS / Modulars etc), never been overcharged before, but guess they have been falling under the radar previously and they are checking box dimensions now.