Not sure, but I would think the JavaScript may be suspect. Try just submitting the form without the javascript to see if it works first, then add the javascript after.
Code:
<form name="pp1" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="add" value="1">
<input type="hidden" name="business" value="me@me.com">
<input type="hidden" name="item_number" value="<? print $name; ?>">
<input type="hidden" name="item_name" value="<? print $codedis; ?>">
<input type="hidden" name="id" value="1">
<input type="hidden" name="amount" value="<? print $fiftyg; ?>">
<input type="hidden" name="amount_base" value="<? print $fiftyg; ?>">
<input type="hidden" name="or0" value="0">
Grams:<br />
<select name="os0">
<option value="None">Please select</option>
<option value="50g=0">50g</option>
<option value="100g=<? $onehundredg_true = $onehundredg - $fiftyg; print $onehundredg_true; ?>">100g</option>
<option value="200g=<? $twohundredg_true = $twohundredg - $fiftyg; print $twohundredg_true; ?>">200g</option>
<option value="500g=<? $fivehundredg_true = $fivehundredg - $fiftyg; print $fivehundredg_true; ?>">500g</option>
</select>
Price:<br />
<input type="hidden" name="undefined_quantity" value="1">
<input type="hidden" name="on0" value="Grams">
<input type="hidden" name="shipping" value="5">
<input type="hidden" name="shipping2" value="7.5">
<input type="hidden" name="currency_code" value="CAD">
<input type="hidden" name="cs" value="0">
<input type="hidden" name="no_note" value="0">
<input type="hidden" name="no_shipping" value="0">
<input type="hidden" name="paymentaction" value="authorization">
<input type="image" src="images/cart.gif" border="0" name="submit" alt="Make Payment with Paypal">
</form>