I can see a huge tickmark icon on the checkout page when viewed from a mobile phone. This appears to be added by your theme, therefore I suggest you also contact GeneratePress support.
For further troubleshooting, it would be helpful if you create a staging site. There you can switch themes and confirm what exactly is creating the tickmark icon.
Please note that we have not received any other reports like this. Also, I cannot replicate this issue on my test site.
This suggests that the tickmark is either added by your theme or a plugin. It could also be a combination, such as added by theme and size enlarged by a plugin’s CSS code.
The best way to troubleshoot this is by performing a Conflict Test. Make sure to clear your cache while performing the test.
I have tried to reproduce the issue using the Astra theme and GeneratePress theme on my test site. But I cannot reproduce the large tickmark.
This suggests it is being created by some other factor. The best way to find the cause of this issue is through the Conflict Test. I recommend you create a staging site and then perform conflict testing.
Here are screenshots of Astra and GeneratePress themes from my test site. The first is Astra and the second one is GeneratePress.