Patreon allows you to pledge a fixed monthly amount. Please note that this is a recurring payment that will be automatically debitted each month. You may set your own contribution.
The PayPal option is a bit rough at the moment. For a one-time donation, click 'Buy Now,' write 'Donation' in the description box, and fill in the amount you want to give. I hope to improve this payment option in the future, but for now, it is what it is.
If you prefer to give cryptocurrency, you can donate to the following Bitcoin wallet: