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 Q1 it was $6.91 billion, up by 77% year-over-year from $3.91 billion. From $14.09 billion in 2019 it increased by 52% to $21.48 billion in 2020.
Source: Amazon Quarterly Results.