There is a free plan for up to 250 profiles. Though forms will feature Klaviyo branding unless on a paid plan. Apart from this they provide the same functions on their free plan.
Additionally, forms that are collecting profiles, it's important to keep an eye on this within Klaviyo. As active profiles do count towards billing. If using a small paid account, I recommend disabling auto upgrades to allow for self-monitoring.
Thank, I have not been able to get the pop-ups to work, only once and for a few moments. I have followed all the recommendations and they do not work. I have PRESTASHOP 8.1.6 integrated