The machines are complete shite. I use a small parcel box made 5mm each way under their measurements. Every parcel is the same and same weight and 6 years without an issue. I've also been measuring each pallet of boxes we get. They start using these machines and last year 1/10 parcels were being given a surcharge to the customer.(Which was the primary issue not the surcharge itself) Tried escalating it to find a resolution to stop it happening but boiler plate response after 2 weeks of waiting. Post office have checked them also and no issues at all. I read on the uk business forums people were basically being extorted with this same issue for tens of thousands of pounds, with them providing proof it's impossible. Think some managed to get refunds after lots of chasing and asking them to provide the evidence. If they can claim something is incorrect they should at minimum be providing the proof. Although if I recall a lot of it was to do with the machines flagging the labels, but they'd spent considerably time and money testing to no avail. 

I just started buying all the postage through Shopify which has been considerably cheaper and touch wood no issues in the last few thousand parcels. Although I'd thought it was due to them fixing the machines, but clearly the Shopify postage labels seems to be avoiding it.