FlexPay now supports Direct Debit payment method for Purchase and One-Time subscriptions. Direct Debit payment method can be enabled in FlexPay options of your website.
Direct Debit (via SOFORT) will then be offered to buyers from supported countries for Purchases and for One-Time subscriptions in EUR. If you do not specify the payment method the buyer will be taken to the payment method selection page.
Documentation for the subscription API:
FlexPay v3.x Subscription
Documentation for the purchase API: FlexPay API v3.x Purchase
Example FlexPay v3 client libraries are available in Control Center for Perl and PHP