The main difference between a Payments Standard checkout and the Express Checkout is that with the Express Checkout you will be able to know the status of the transaction when it is placed. With the regular Payments Standard you do not get to know the status of the transaction when it occurs. Only after the transaction occurs will you be able to know the status by setting up an IPN page. This is more difficult and takes much more time to integrate. If you wanted to add the regular Payments Standard you could do so with the following steps:
Clear out the checkout page or creating a new blank one. On the new page from the eCart insert bar choose checkout form. Then select 'remote' and chose Website Payments Standard. This will give you an interface where you can setup the standard PayPal checkout. You will need to supply the PayPal email address for the profile instead of API credentials.
I think it would be better to use the Express Checkout option. I'm not able to reproduce the issue as you are seeing it. Please post back with your checkout pages that were crafted minus the sensitive details like the PayPal api username and sig. I will check on things to see what result you are getting and let you know what I come up with. If this issue requires further investigation I will open a ticket for you on this issue.