In order to settle the payment at the convenience store, you do NOT need the physical Smartpit card.

Kindly be assured you can find your Smartpit number via the [Sakura Mobile My Page].

First, log in to your [Sakura Mobile My Page] account. 

LINK: https://myaccount.sakuramobile.jp/auth/login

If it's your first time to log-in on the My page website, kindly refer to this article on How to log in to [Sakura Mobile My Page] for the first time?

1- After you log in to your page, kindly click on the three dots on the upper right as shown in the below picture:

2- From the menu, please click on (Account)

3- From the Account page, you can see your personal information, as well as the Smartpit number.

Please refer to the following links on how to pay at convenience stores:
Family Mart: 
Lawson/ Mini Stop

Kindly note you can NOT use 7/11 convenience stores to pay the bill. Thank you for understanding.