The definition of payola is: the practice of bribing someone in return for the unofficial promotion of a product in the media. In the context of music blogging, this means accepting money for coverage without disclosing that money was paid.
Disclosing that your review was part of a promotional budget is an important way to ensure that your reviews are not considered payola. Simply let your readers know that your review was is sponsored content and both parties should be covered.
While preparing your deliverable you'll see an option to customize your disclosure. The text can be customized. Some suggestions include:
Feel free to discuss the specifics of this disclosure with your client while working on the project.