Re: borders to be regularised or smoothed out by mutual agreement
Customs are separate to Schengen. The customs can stop anyone anywhere in the Country (this was introduced with Schengen). This can mean if you're naughty and don't declare your shopping or other stuff then you get fined or even prosecuted in extreme cases.
The reality though is that people bring things over the border all the time and regard the occasional fine as "worth it".