Is it permissible to receive a sum of money from a service provider to join or remain with them, to the exclusion of other competitors?

There is no objection to taking the amount paid by the service provider for dealing with them and preferring them over others. This is considered a promotional gift offered to attract customers, and it can be regarded as compensation for not purchasing from others. You may take it, provided you adhere to the condition of exclusively purchasing the service from them.