FreeUP's penchant for removing, adding and changing things in people's accounts without any explanation given is truly disconcerting and unacceptable, and they're hurting their chances of doing well because of it. For a while, I was taking screenshots, which is a hassle and shouldn't be needed, but stopped when FU became more consistent for me. I just bumped one account to the $15 plan because my coupon earnings have been very consistent. Yet I'm still treading on pins and needles anticipating another FreeUp sleight of hand snafu.
I had a similar incident last month when it dropped close to $2, overnight. I wrote to customer support about that and have not had a reply yet. I wrote both by using the form on their website and by reply to a customer service communication I had with them last fall.
I have been with them for nearly a year and the times I have heard back from them, it took several weeks to get a reply.
I have about $20 in rewards that will be used to pay the $15 due in two days. I was able to do the same when upgrading from the free plan to the $10 plan. The CC I used to purchase the original FU sim is still on file in my FreeUp account but hasn't been charged since purchasing the sim. So, it would appear if one has enough rewards to pay for the $15 plan, then no CC would be needed, and if rewards are insufficient to pay the full $15, then I assume FU would use all one's rewards and charge your CC the difference.