HFSLIP is "free", but if you still want to say thanks to the developers, here's one way to do it.
Donation is executed on a completely voluntary basis, and you're free to choose the amount you wish to pay. The money will be split evenly across the accounts of the main developers.
One thing... The Paypal account that's being donated to is "unverified". This is because no credit card is linked to the Paypal account.
Thanks for your donation.