Hi, @seamustully thanks for your post and welcome to the community.
You can ask your designer for their email and add them as staff in your Shopify with limited access. Your bank details will be safe and hidden from your staff.
How to add someone to Shopify as a Staff:
And as being a professional Shopify Developer/Partner I am also available to redesign and redevelop your website from scratch if interested so feel free to mail me anytime. Thanks and Best Regards