I think you have given wrong access rights. Portal user(your customer) have only Portal access rights then your portal user can see own documents(sales, invoices,...) under mydomail.com/my . 

If you are giving access like sales: own documents that means he/she is not a portal user but your regular user who can do different operation in your system like validate oppo..

The problem was that the user was not a follower of his invoice and the default rule in

Groups/Other Extra Rights / Portal  is:

Portal Personal Account Invoices: [('message_partner_ids','child_of',[user.commercial_partner_id.id])]

So it won't apply to the owner of the invoice if he is not a follower. (why not?)

Adding a rule like [('commercial_partner_id','in',[user.partner_id.id])] solves the issue