If you use SagePay and Operate's Member Portal, you have the option of taking online payments from your customers. The first time a customer pays an invoice on the Oportal, they have an option to tick for the card to be used for future payments. They can also pay one time, for example for bookings or invoices, without activating recurring payments.
There are a couple of things you need to do to make this work - both for one time and for recurring payments.
You need to have Continuous Authority enabled by SagePay on your Merchant Account (you will need to contact SagePay to enable this).
Ask essensys to enable tokenisation.
Test tokenisation with a test account.
Add the following IP addresses into your SagePay account:
SG 188.8.131.52 + 184.108.40.206
US 220.127.116.11 + 18.104.22.168
EU 22.214.171.124 + 126.96.36.199
For help on adding IP addresses to your Account, please see the SagePay specific guide.