An API (Application Programming Interface) is used in many different contexts in this industry. To clear this up with our service read below:

We offer a Common Gateway Interface (CGI). This means your site will need to contain an HTML form or link with cgi parameters appended which passes the correct parameters (and your customer) to our site for checkout. Passing in the Proper Parameters will result in your customer going through the rest of our checkout process (such as entering their credit card information), and after verification, a successful sale.

All parameters are discussed in detail within the Secure Admin Panel for Vendors.

As far as we know, you cannot use our services with a Yahoo! store because of the way they have choosen to set up their payment processing gateway. We are not compatible with their gateway services at this point in time.

You may be able to find a 3rd Party Shopping Cart that can meet your needs.
http://www.2checkout.com/community/blog/category/knowledge-base/tech-support/3rd-party-carts/recommended-carts/

Due to current credit card regulations, 2Checkout is no longer able to allow the purchase routine to be loaded with in a frame set. By loading our site in a frame, the customer would remain on the suppliers server while navigating though out purchase routine. Because 2Checkout is an authorized reseller and not a payment gateway or payment processor it is required that customers are aware that they are ordering from us at the time of purchase. To correct this the 2CheckOut transaction pages will break out of a frame set if they are loaded inside of one. Attempting to load the 2CheckOut transaction pages within a frame set will prevent them from working as intended and will generate a parameter error. If your site uses frames you may still integrate with 2Checkout by using target=”_blank” to load the 2Checkout purchase routine in a new window.

Example of an HTML form loading in a new window.

<form action='https://www.2checkout.com/checkout/purchase' target='_blank'
method='post'>
<input type='hidden' name='sid' value='123456' >
<input type='hidden' name='product_id' value='101' >
<input type='hidden' name='quantity' value='1' >
<input name='submit' type='submit' value='Buy from 2CO' >
</form>

Example of a purchase link loading in a new window.

<a href="https://www.2checkout.com/checkout/purchase?sid=123456&quantity=1
&product_id=101" mce_href="https://www.2checkout.com/checkout/purchase?sid=123456&
quantity=1&product_id=101" target='_blank'> Test Product </a>

Each button added to the page needs to have its own opening and closing form field. This tells the browser that each form is separate, and not part of the same one.

The following correct example generated two separate buy buttons:

<form action="https://www.2checkout.com/checkout/purchase" method="post">
<input type="hidden" name="sid" value="XXXXXX" />
<input type="hidden" name="product_id" value="10" />
<input type="text" name="quantity" size="5" value="1" />
<input type="submit" name="submit" value="Buy from 2CO" />
</form><form action="https://www.2checkout.com/checkout/purchase" method="post">
<input type="hidden" name="sid" value="XXXXXX" />
<input type="hidden" name="product_id" value="10" />
<input type="text" name="quantity" size="5" value="1" />
<input type="submit" name="submit" value="Buy from 2CO" />
</form>

** Notice that there are two opening tags ( <form> ) and two closing tags ( </form> ).

Another possibility, if you are using an HTML editor, is the editor may be adding extra numbers to the parameters, thinking each identical one is part of the same form, and not a separate form. Look at the parameters in the forms, and make sure there are not extra numbers added to it.
Incorrect Example:

<form action="https://www.2checkout.com/checkout/purchase" method="post">
<input type="hidden" name="sid1" value="XXXXXX" />
<input type="hidden" name="product_id" value="10" />
<input type="text" name="quantity" size="5" value="1" />
<input type="submit" name="submit" value="Buy from 2CO" />
</form>

** Notice that the in second line from the name=”sid1″, this should always be name=”sid”

Important Note: When you are testing parameters, please be certain to clear the cookies in your browser between tests to ensure the changes you have made are applied. For more information on clearing the cookies please click here.

Yes.

Because our Plug n’ Play cart functions through the use of standard HTML forms, you can use the PNP cart in any website creation utility that supports HTML forms, including Macromedia’s Flash product.