The only way to pay is through PayPal? Can I pay with a credit or debit card instead?
You don't need to have a PayPal account to pay for the Fertility Calendar.

In this case select Pay with a debit or credit card instead of login to PayPal account...

...and then input credit card information.

Note, that PayPal is a solid company with a strict security regulations and paying via PayPal is safe.