M365 Licensing for small retail store

I have a client who is a small retail store, using generic email addresses based on department, as opposed to individual users. Looking for the best way to license for Office Desktop Apps. 

Scenario:

Front counter - 2 x PC's and both need access to generic email address sales@ 

Back office - 1 x PC needs access to generic email address admin@

For us to have Office on these machines, using M365 is the correct way: 

Option 1:

Front Counter: 

1. Create Business Standard License and email address sales@
2. Install Office on both machines, as Business Standard gives up to 5 copies of Office

Back Office:

1. Create Business Standard License and email address admin@
2. Install Office on both machines, as Business Standard gives up to 5 copies of Office

Option 2: 

1. Create 3 x Business Standard licenses, one for each PC - although the associated email addresses will never be used and be hidden
2. Create a shared mailbox sales@ and admin@
3. Assign permissions for shares mail boxes as needed - the shared mailbox would need to be the default Send From email addresses

I am getting conflicting advice - Option 1 is not legal as the mailboxes are not actual people, and Option 2 seems clunky

Business does not want to purchase once off copies of Office due to wanting to pay monthly... 

Welcome people's thoughts.