Other sales is assumed to mostly consist of advertising (used by sellers and brands). It also includes other things like cobranded credit card agreements, but those are expected to be a minor part.
Amazon defines this as:
Includes sales not otherwise included above, such as certain advertising services and our co-branded credit card agreements.
2014: $1.322 billion, 2015: $1.710 billion, 2016: $2.950 billion.
Last reported quarter 2021 Q4 it was $10.43 billion, up by 31% year-over-year from $7.95 billion. From $21.48 billion in 2020 it increased by 55% to $33.34 billion in 2021.
Source: Amazon Quarterly Results.