I doubt that there will be a (food) rejection for overweight.(!?) ([lack of]Profitability maybe Yes)
Never had the misfortune of dealing with some of the retailers / brands in the UK, Charles?
I while ago I dealt with a startup, and in getting used to their somewhat basic line they erred towards over- rather than under-filling, which seems sensible to normal humans, but alas it resulted in complaints and rejections for overfilled packs...
See below---the allowable variance for your pack size is a MAX of 4.5 g
(Text snipped for brevity)
If the OP is importing product to Europe then it's worth noting that the weights and measures at the EU level is a Directive, and is therefore implemented in local regulation in each member state. This means that regulations in specific countries can vary slightly.
Nonetheless the maximum allowable variance is actually twice the tolerable negative error (TNE), i.e. in the case of the OP's product the absolute minimum weight for a pack would be 65 - (4.5*2) = 56g. There are of course statistical constraints on the number of packs that fall into the TNE / 2TNE below the declared weight region, although no such constraint on being above this - the regulatory framework doesn't have any problem with food businesses giving away free product above the declared quantity.
Obviously this is a very UK-centric take (derived from the EU directive, before the "B word" happened) but if you are dealing with exports into this market then this guidance gives a reasonable overview: https://assets.publi...ons-2006-tp.pdf