Customisable ways to offer discounts?

I've seen there are so many basic ways to offer discounts that are missing... how do you guys deal with that?

For example:

• I want to give a discount to all owners of Product A for product B
• I want to give all current clients (who created an account) a discount
• I want to prevent discounts to be applied on items already on sale.
•  I want to send a discount code only to people who have purchased in the past with AND are subscribed to the newsletter (for legal reasons)

1 minute ago, Alejoca said:

• I want to give a discount to all owners of Product A for product B
• I want to give all current clients (who created an account) a discount
•  I want to send a discount code only to people who have purchased in the past with AND are subscribed to the newsletter (for legal reasons)

You can use Squarespace Campaigns to send a discount code to customers who own product A, have created an account or who have purchased in the past. 
 

2 minutes ago, Alejoca said:

I want to prevent discounts to be applied on items already on sale.

It isn’t possible to prevent discounts applying to sale items. 

1 minute ago, paul2009 said:

You can use Squarespace Campaigns to send a discount code to customers who own product A, have created an account or who have purchased in the past. 

yes but:

1. AFAIK, people who know the code (i.e. if it's shared) can also use it just by having an account, am I wrong?

2. Also, I cannot send an email to someone who owns a product and has subscribed to the newsletter, I have to manually make the list of those people. As per EU regulation I cannot send emails to people who explicitly have subscribed to a newsletter, so emailing "everyone who owns a product" can be very illegal.