Shop Pay can be used during check out (if you qualify), just login to your Shop Pay account at the checkout page and you will be redirected to confirm your order details. If you have any questions about your Shop Pay account, please head to their help page for more assistance.
Shop Pay offers installment payment options, however that varies from account to account. We recommend to check out the Shop Pay Installments Help Center and they will be able to further assist you with any other questions. The option should be presented as you confirm your order details before you hit payment.