Yes indeed we can help you maintain and develop your Shopify website.
We are a Shopify Partner so you will not need to share your username and password with us. Just contact us and let us know what you need done and include the development link to your site that looks something like: name.myshopify.com
We can then request access and you will receive a notification from Shopify where you can approve our request for access.
Once this is done we can log in and help you with any issues that you are having or send you a quotation for the work for your approval.
Shopify Partners do not count toward your staff limit. You can also limit our access to only certain parts of your site i.e. you may not want to share customer information or orders etc.