You should be able to adjust the decimal symbol in the cart object when you edit it. If you have done this part and still have this issue then you may want to try updating the code so that it uses the number format instead of the money formatting like this:
from this
<%=WA_eCart_DisplayMoney(FrogStyling, WAEC_VerzendingskostenEnvelop())%>
<%=WA_eCart_DisplayMoney(FrogStyling, WA_eCart_GrandTotal(FrogStyling))%>
to this
€ <%= FormatNumber(WAEC_VerzendingskostenEnvelop(), -1, -2, -2, -2) %>
€ <%= FormatNumber(WA_eCart_GrandTotal(FrogStyling), -1, -2, -2, -2) %>