Anytime a creator accepts collaboration terms and has received product from the brand, regardless if payment is involved, they are required to follow all collaboration guidelines and post by the post date.
However, we understand not all products are meant for everyone and creators should always remain authentic to their followers. If the product ends up being incompatible to your brand or you experienced a bad reaction, definitely let the brand know so they can provide a return label and cancel the collaboration. Lack of communication will lead to the brand reporting you and possibly suspension.
Keep in mind, each campaign detail page indicates whether or not payment is involved.