Recurring monthly donations is known as sponsoring, and you can set up your sponsorship here: http://www.linuxmint.com/sponsors.php
Thanks for considering to sponsor Linux Mint! Awesome
Note that with very small payments PayPal takes the full amount as fee for their services (for each transaction PayPal keeps up to 3.9% of the transaction amount + a fixed amount dependent on the currency used, but at least $0.30). So if by small recurring donations you meant a few bucks, a once per year donation for the total amount would contribute more to Linux Mint.