Table:
|  | For the Year Ended December 31, |  | Change |  |
| --- | --- | --- | --- | --- |
|  | 2018 | 2017 | Amount | Percentage |
|  |  | (dollars in thousands) |  |  |
| Revenue: |  |  |  |  |
| Subscription and returns | $254,056 | $199,942 | $54,114 | 27% |
| Professional services | 18,042 | 13,217 | 4,825 | 37% |
| Total revenue | $272,098 | $213,159 | $58,939 | 28% |
Paragraphs:
Year Ended December 31, 2018 Compared to Year Ended December 31, 2017
Revenue
Total revenue for the year ended December 31, 2018 increased by $58.9 million, or 28%, compared to the year ended December 31, 2017. Subscription
and returns revenue for the year ended December 31, 2018 increased by $54.1 million, or 27%, compared to the year ended December 31, 2017. Professional
services revenue for the year ended December 31, 2018 increased by $4.8 million, or 37%, compared to the year ended December 31, 2017. Growth in total
revenue was due primarily to increased demand for our products and services from both new and existing customers. Of the increase in total revenue for the year
ended December 31, 2018 compared to 2017, approximately $29.6 million was attributable to existing customers, approximately $28.3 million was attributable to
new customers, and approximately $1.1 million was due to interest income on funds held for customers. Total subscription and returns revenue for 2018 included
$1.2 million related to our cross-border transactions technology acquired in May 2018.
Question:
What is the increase in total revenue attributed to?