One suggestion is to use the subscription command and it's associated variables, below is one example of how this can be done:
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<title>Donation Subscripton Example</title>
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<meta http-equiv="Content-Language" content="en">
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Donation Examples
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Make a One Time Donation or setup a Donation Schedule like a Subscription.
<br><br>
<!-- Start of Form -->
<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick-subscriptions">
<!-- Replace "business" value with your PayPal Email Address or Account ID -->
<input type="hidden" name="business" value="your email address">
<input type="hidden" name="item_name" value="Widget Fund Donation">
<input type="hidden" name="item_number" value="WFD-1002">
<input type="hidden" name="no_note" value="1">
<!-- no_shipping value 2 needed to send the free gift -->
<input type="hidden" name="no_shipping" value="2">
<input type="hidden" name="tax" value="0">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="button_subtype" value="products">
<input type="hidden" name="no_note" value="0">
<input type="hidden" name="bn" value="PP-SubscriptionsBF">
<!-- Replace value with the web page you want the customer to return to after a successful transaction -->
<input type="hidden" name="return" value="http://www.yourwebsite.com/ThankYou.html">
<!-- Replace value with the web page you want the customer to return to after item cancellation -->
<input type="hidden" name="cancel_return" value="http://www.yourwebsite.com/Cancel.html">
<!-- -->
Enter the amount you would like to donate:
<!-- a3, Required, Regular Subscription Price -->
<input type="text" name="a3" size="4" maxlength="7">
<br><br>
<!-- src, Optional, Recurring Payments, 0 no recur, 1 payments recur -->
<input type="hidden" name="src" value="1">
<!-- -->
Select the length of the desired Billing Cycle:
<!-- p3, Required, Duration based on the value of t3 -->
<select name="p3">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="15">15</option>
<option value="30">30</option>
</select>
<br><br>
<!-- -->
Select how often you would like to Donate:
<!-- t3, Required, Subscription Units D W M Y, -->
<select name="t3">
<option value="D">Day(s)</option>
<option value="W">Week(s)</option>
<option value="M">Month(s)</option>
<option value="Y">Year(s)</option>
</select>
<br><br>
<!-- -->
Select the number of Donation Payments you would like to make:
<br><br>
<!-- srt, Optional, Recurring Times, must be used with src -->
<select name="srt">
<option value="">Until Canceled</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="6">6</option>
<option value="12">12</option>
<option value="18">18</option>
<option value="24">24</option>
<option value="36">36</option>
</select>
<!-- sra, Optional, Reattempt on failure 2 more time, value 0 or 1, if 0 no reattempt -->
<input type="hidden" name="sra" value="1">
<br><br><br>
Thank you for your Donation,
<br><br>
<!-- -->
<input type="hidden" name="bn" value="PP-SubscriptionsBF:btn_donateCC_LG.gif:NonHosted">
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_donateCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
<img alt="" border="0" src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>
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<br><br>
<br><br><br><br>
<hr align="left" width="50%" noshade>
<br><br>
NOTES:
<br>
In order to test the code, you must replace the "business" value variable with your PayPal Email Address or Merchant Account ID.
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