On 25.10.16 I reported an Amazon Locker (a locker from which consumers can have their Amazon orders delivered) which was listed under the "Department store" category on the Google Map at it's address at the Co-Op shop (a mini-supermarket) on London Road, Guildford, Surrey, UK to Google. The listing was removed by the day after. I was unware of this location - this means that in Guildford there will soon be 5 Amazon Lockers!
"Express mail dropboxes" are permitted to be be listed on the Map. Looking at some local Locker listings it seems that most Lockers in the South East UK are listed under the "E-commerce service" category but there is not a consistent categorisation. As to what the 'correct' categorisation should be, a Google My Business representative on Twitter informed me it should be "Mailing Service".