At the moment, QuickBooks Solopreneur does not support customizable letterheads on invoices or estimates, Taylor. However, there is a way available for you to handle your customer payment information.
Since you can't add your account and routing numbers directly to the letterhead, you can include this important information in the Customer payment options or Notes to customer sections of your invoice or estimate. This will ensure your clients know where to send their payments.

Moreover, if you want to make your sales forms even more personal, consider exploring upgraded plans. The QuickBooks Plans and Pricing page can help you find the right fit for your business.
Additionally, to learn how to record invoice payments, please refer to this resource: Record payments in QuickBooks Solopreneur.
If you have any questions or need a hand with anything else, just let me know. It's always my pleasure to help. Have a great day!