I think the issue is the slash. You might try URL-encoding it. Try: cart%2Fcheckout
Also, just FYI, how are your Cart Links being created? You should use the l() function. The fourth parameter in the l() function is the query (anything after a "?" sign). So yours would be..
<?php
print l('Buy Now', 'cart/add/p128_q1-ipersonal_one_year', NULL, 'destination=cart/checkout');
?>For more check out the API page: http://api.drupal.org/api/function/l/5
You can also have other parts of the cart link created automatically by using info from the node, such as $node->nid (for the one they are buying). Hope this helps.