Overriding price for non dues subscription product in cart

We have successfully updated the pricing in the cart for different items in the past by setting the 'IsUnitPriceOverridden' flag to true, but that doesn't seem to work for non dues subscription products (product type: SUB). Is there a trick to this? We tried updating the price on the item already added to cart by the user as well as adding the item with preferred pricing ourselves via code without success. Added the override flag to order lines, child order lines and wherever possible but no luck. :(

This is for a client on iMIS v2017 so we tried both via API and SOA's without success. We tried to build the order and add to cart as described here "https://developer.imis.com/docs/purchasing-non-dues-subscriptions#4-create-an-orderline-containing-the-product-details" to see if we could control pricing with this but looks like the 2017 version doesn't seem to recognize 'LegacyMembershipItemData' and fails with an internal server error when adding the order to cart.

Any ideas? Please and thank you!