The easiest, Cheapest (FREE) way to do it is...
use hotmail calendar... you can set up an email reminder to go out up to 7 days before paypal takes it's money,,, and don't quote me on this, but I am wondering if you can put more then one email in the email to get notified option? Maybe a comma after the first and add another will send a reminder to both emails? you will have to test that, but, worse case, you can create double, one for your client, then one seperate one for you.

just under the section that you put your email in to recieve the reminder, there is an option for recurring.
Set that to every 30 days, and dude, your good.

I have my clients all pay me around the same time, so I would only have to set myself up with one reminder, and then each of them set up to recieve their own reminder each month.

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Tony V Get Paid To Blog Even If you don't Blog