There are two payment options available for Kantar Marketplace projects.  You can purchase credits in advance or pay as-you-go invoicing. 

Invoicing: Pay via invoice for launched projects. This option works best for ad hoc, occasional usage. This will be the default payment your account is setup with in Kantar Marketplace. 

You will see the Invoice tab upon selecting "Proceed to Payment".

Credits: Purchase credits up front in “blocks.” This option is perfect for teams that share a common budget or to manage costs month-to-month.  1 credit = 1 dollar in your local currency.   

If you have previously purchased credits or a credits package with Kantar Marketplace, you will see the Credits tab upon selecting "Proceed to Payment"

You will see both Credits and Invoice as Payment options if you have requested for both options to be setup.