After the free trial
If you signed up before October 14, 2014, you won't automatically be charged after your free trial has ended. You'll have the option to upgrade to Express membership. If you choose not to sign up for membership, you can continue to shop with Google Express on a pay-per-order basis.
If you signed up after October 14, 2014, once your free trial membership expires, you'll be automatically enrolled in the payment plan ($10/month or $95/year) you selected when you signed up, unless you choose to end your membership.
Of course, if you have any questions, our Customer Support team is happy to help.