I would do it like this:

var nextStep = "url from umbraco node for the next checkout step";
            if (basket.OrderLines.Any(x => Product.FirstOrDefault(y => y.Sku == x.Sku).ProductDefinition.Name == "membershipdefinitionName"))
            {
                nextStep = "alternative url for membership information entry";
            }

So you make a "optional" checkout step, and the content of the basket defines the next step in the checkout flow.

I hope this helps

I have never tried to make a website with ucommerce.. Where should i put this code? how do i make the checkout?

The url's you are refering to, where do i get the url to "the next checkout step" and "alternative url for membership information entry"? is it normal pages i should use as checkout?

Yes, its normal pages you use for the checkout.

I make a "Checkout" content section like this:

Checkout

1. Basket
2. Addressinformation
3. Shipping and payment method
4. Order confirmation
5. Order Complete confirmation
6. Order Declined

var currentNode = uQuery.GetCurrentNode();
var nextCheckOutStep = currentNode.Parent.GetChildNodes().FirstOrDefault(x => x.SortOrder > currentNode.SortOrder);

This will return the next step in the checkout flow  (in my example the Addressinformation node).

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