According to FTC, anytime you create a “material connection” with a brand and post about their products, you must clearly disclose this connection with this brand in your posts or videos. This includes monetary payments from the brand but also, and this is where it’s important, the provision of free products.
Bottom line: you must let your audience know that you got these products for free.
You must do this for every collaboration and everytime you post about a product. It’s not just about respecting the law, but also about being honest with your audience.