In Amsterdam, and in particular the historic centre, parking is expensive. On the other hand the accesability by public transport is very good, and the venue is just one bus stop, not even a ten minute walk away from Amsterdam Central Station. (See How to get there)
If you need to travel by car you can use parking garages within walking distance of the Posthoornkerk. For example;
Vicky Parking, Tussen de Bogen 56, 1013 JB : 6 minutes walk

Parkeergarage Willemspoort, Haarlemmer Houttuinen 549, 1013 GM : 6 minutes walk 

ParkBee Havengebouw, De Ruijterkade 7, 1013 AA : 10 minutes walk

Parking Service, Prins Hendrikkade 15, 1012 TL : 6 minutes walk

Q-Park, Prins Hendrikkade 20A, 1012 TL : 8 minutes walk

Parking Service, Noordermarkt 42B, 1015 NA : 6 minutes walk

Jordaan Parking Brouwersgracht 230, 1013 HE : 5 minutes walk

You could also park for less at a garage more on the outskirts, like Parkeergarage P6 (Pathe, Johan Cruijff Arena) De Corridor 15, 1101 BC and from there take the subway, or (rental) bike.