As the merchant and seller of record, FastSpring purchases products and services from you - the publisher / creator / provider - and re-sells the products or services to the end customers. Our status as merchant and seller of record provides many advantages to you, such as allowing us to handle all collection and remittance of sales tax and VAT, so you do not have to deal with those issues. It also means that any terms and conditions associated with a payment transaction processed by FastSpring need to be FastSpring's own terms and conditions (as opposed to your terms and conditions or your End User License Agreement).
Therefore, we recommend that you include any necessary terms and conditions for using the product - and / or any EULA - either on your website, or else perhaps in the installer of your products where applicable.
The following order flow illustration may be helpful.
Although it makes sense to include your terms and conditions on your website, you should consider carefully the potential impact on conversion rates if you require customers to accept the terms prior to purchasing. Every additional page displayed to a potential customer and every click you require the customer to perform is another opportunity for the customer to abandon the process without purchasing.