No problem! To be clear, a subdomain such as send.storename.com is NOT the same as a username at your root domain, such as hello@storename.com. In the example you gave, hello@storename.com is only using your root domain of storename.com.
Email domains (both root and subdomain) are in part what ISPs use to measure your sender reputation since it informs where messages are coming from. Your sender reputation has a direct impact on your deliverability. Since ISP’s don’t share exactly how they decide whether a domain is trustworthy sender or not, it’s strongly recommended to minimize variables using subdomains.
Using a subdomain like send.storename.com effectively separates emails you’re using for marketing purposes from other activities on the root domain. For example, if you were only using a root domain and you got an increase in spam reports on a marketing campaign, the impact to your sender reputation would affect deliverability of ALL emails using the root domain, such as sales outreach, customer support (hello@storename.com), or even employees’ ability to email each other.
This is why you definitely want to separate your marketing communications with a subdomain. Let me know if that helps :)