The postage costs for forwarding papercrush pop up cards within Germany currently vary between EUR 1.00 (compact letter) and EUR 1.60 (capital letter).
Cards with a dimension of 12x17,5 cm require a postage stamp franked with EUR 1.00, cards with a dimension of 13x18 cm or 15x15cm require a postage stamp franked with EUR 1.60,
You can also find an overview of Deutsche Post’s current postage costs here: https://www.deutschepost.de/de/b/briefe-in-deutschland.html