If i only use mobile app, do i need to pay?

The mobile app is one of the features, not a stand alone product.

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If the subscription cost is our of your budget, consider migrating to another accounting app with a lower fee.

I can share some information in managing your account, @pikachuqqq171717.

Yes, you'll need to pay for a subscription to use QuickBooks Online (QBO). The information you've entered in both the mobile app and the web will automatically sync. To check which plan is best for your business and how much it will cost, check out this link: Plans & Pricing. 

From here, you can now log in to your account on the web to subscribe. Follow the steps below:

1. Log in to your account using this link: QuickBooks Login. 
2. Go to the Gear icon (⚙) and choose Account and Settings under Your Company.
3. Select Billing & Subscription, enter the updated billing information.
4. Click on Subscribe.

For more information about subscribing, changing your subscription, or just viewing your billing history in QBO, refer to this article: Manage billing, payment, and subscription info in QuickBooks Online. 

Here are some video tutorials that can help you get started with QuickBooks Online: Video tutorials for QuickBooks Online.

I've also included an article that will guide you on which QuickBooks Online mobile app features are supported on your mobile device: Compare mobile app features.

Keep me posted if you need additional help in managing your subscription and or anything else, by leaving a reply below. I'll be right here whenever you need a hand. Have a nice day!