Our cart is the only format in which we can offer the full range of our discount features, so if you wish to offer global discounts that can be used on any product you would need to use our recommended Add to Cart and View Cart buttons. Those normally contain a PayPal button for the checkout process, but that can be customized:
Buy Now buttons can only accept discounts which are specific to that particular product and which do not use any qualifying conditions such as expiration dates or minimum amounts or limited uses, etc.
The good news is that you can use the same code over and over with different discounts. So you could get the result you are looking for by making a separate discount for each individual product that discounts the amount equivalent to 10%, and assigning the same discount code to each of these so that the buyer does not need to remember more than one code when purchasing.