I wasn't there when PayStand was installed/implemented on our NetSuite account, but ours didn't come with the ability to add a Pay Now button for Sales Orders in SuiteCommerce MyAccount, only Invoices.
However, it DID have a couple of buttons on the Sales Order in the regular NetSuite UI for, for example, sending an email with the Pay Now button in its content, and when the recipient clicked on it, it would attribute their payment to the Sales Order. By the time I'd gotten to it, some customization beyond the normal PayStand install had already been done, though, so I don't know if it's a standard feature. I didn't really have anything to do with the install. I just fiddled with it to get it to put a Pay Now button in more places in SuiteCommerce than it normally does.