"a word from us to the Custom House people"

The Custom House was the place where goods entering and leaving the UK were checked and any necessary tax paid. As the pornography laws on the Continent were more relaxed than in nineteenth century Britain, the photographs and books that Mr Verloc buys in from Paris and Brussels are likely to contravene the censorship laws and, as such, could get him into trouble with the law or cost him a heavy fine if opened by customs officials.