You should be able to use expr="//t[contains(@t-value, '/shop/checkout?express=1')]"​, your code wasn't working because the t-value field contains '/web/login?redirect=/shop/checkout' if redirect_to_sign_in else '/shop/checkout?express=1'​, not just the part you were trying to match on (ie. at the moment the xpath is being handled, it's still looking at that entire value as just a literal string.)

You can use 

expr="//t[@t-set='redirect_url']"