As an agency, in order to add the Shopify account of your customers, you need to have the following permissions on their Shopify:
The most important one is "Manage and install apps and channels"
Then
“Home”
“Orders” => “Edit orders”
“Draft orders”
“Products”
“Gift cards”
“Customers”
“Reports”
“Dashboards”
“Discounts”
You can learn more about Shopify permissions here.