Not necessarily - the WTO only define the first 6 digits to be globally recognised, all others are importing country specific. Some countries only recognise 8 digits e.g. Norway:
tolltariffen.toll.no
Some support 10, but that doesn't mean they are the same as the UK, EU etc. Take the USA for example:
hts.usitc.gov
In this case, USA (and also Norway as it happens), your 3919108085 isn't recognised by any of them as the "80" subheading is not defined. In reality, you'd hope an importing customs office would just ignore anything it didn't understand, so your 3919108085 would become 391910 in these cases. It may mean a marginal change in duty rate for the USA case (6.5% or 5.8%), but no difference at all for Norway (0%).
It depends - our integration can change the code depending on destination - to the EU we use the full 10, others may just get the default 6, Switzerland may get 8 (but here again the 7&8 digits aren't necessarily same as an EU 7&8). It probably makes little difference for small value items and no one wants overhead - having too many different codes on a courier order is bad news as it will typically incur multi-line entry fees (can be €5 a shot for EU countries with UPS/DHL etc) for each item line over 5 with a different code/origin combination. Royal mail (and parcelforce on the non-express services I believe) don't (currently) have a line limit.
Chances are, in your envelope case for a single(?) decal, no duty is relevant as its value is under the import threshold so not worth stressing on nailing down a too specific code that someone could take a different view on in any case (or maybe just ignore). It's probably worth it on high value items / order combinations, which is where we started to come unstuck originally, so now try and make it recognised for the recipient.