Subscription enrollment covers most items. Depending on what tier of subscription you're enrolled in, you may have some a la carte items in an order, such as authoring a Safety Data Sheet (SDS).
A la carte items, such as additional reports or documents that you may purchase during your product's registration, will be put into your shopping cart along with your registration. For items that are a la carte, you may select a different payment method for that line item. The payment method is picked from your registered payment methods in My Account, Manage Subscription, and may be different than the payment marked as your default.