Ting for international unlock - cheapest way to do it? referral?

I have a couple of phones I'd like to unlock for international use and I believe only ting do it for sprint mvnos. Do correct me if there is another to consider!
So assuming I have to temporarily sign up for ting - is there a way to get the first month free? How does the referral credit work - will I receive it to use for the first month? Or am I looking at spending $6+ tax min to do this.

$6 isn't so bad since i have a few to unlock.

Can't believe I want to do this...