OK....My educated guess was a little off the mark; but not too far!

You set up your shipping calculator in your Merchant Service page, (Shipping Calculator). Then you create your PayPal button at PayPal. Be sure to put a weight figure where it asks for weight. It will say (optional), but you need to put a figure in there. Doesn't have to represent the actual weight of any item, but must be there to generate the weight code in the button!

Then you just copy that code as many times as you need and edit it for each item.

Example:
<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="image" src="https://www.paypal.com/en_US/i/btn/btn_cart_SM.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
<img alt="" border="0" src="https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
<input type="hidden" name="add" value="1">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="business" value="xxxxxx@ipscompany.com">
<input type="hidden" name="item_name" value="Test Item">
<input type="hidden" name="item_number" value="12345">
<input type="hidden" name="amount" value="29.95">
<input type="hidden" name="no_shipping" value="0">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="weight" value="1.21">
<input type="hidden" name="weight_unit" value="lbs">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="bn" value="PP-ShopCartBF">
</form>

Just change the codes in red to match each item you are selling.