Unfortunately, without payment, influencers cannot be guaranteed to post. It is up to the influencer if they believe the product is an authentic fit for their feed and that they can genuinely promote your product or service.
Obviously, they have applied to collaborate with you, so as long as your product/service matches your brief and you have set realistic deliverable, both parties should be happy to complete.
If they don't wish to post, they will send your product back to you. Make sure you are clear in your product descriptions so influencers know what they are receiving.
Don't forget, you don't need to ask for in feed posts (which many influencers only complete for paid work) but you can also use instagram stories.
Remember, all influencers booked on contra jobs you get to leave a rating and review for. This is a great way to remind them to do their posts in a timely fashion so you can give them a great review.