By default, Charitable will accept any donation over $0.

You can customize the minimum donation amount with a little bit of code, using the charitable_minimum_donation_amount filter:

add_filter( 'charitable_minimum_donation_amount', function() { 
    return 2;
} );

The above code sets a minimum donation amount of $2; any donation of $2 or more will be accepted.

Permitting $0 donations

As mentioned before, Charitable will by default allow any donations over $0.

If you would like to also allow $0 donations, you can use the charitable_permit_0_donation filter:

add_filter( 'charitable_permit_0_donation', '__return_true' );