There are many things which I find annoying about shopping online, the most irksome generally deal with grocery shopping. Now let's assume you're an online grocer and you've covered all the basics of having stock, delivering what you say you're going to deliver, the produce being in a reasonable condition and it arriving on time (a tall order for some).
Then, my advice to you is to apply what I call the Wardley Happiness Equation of Extreme Exceptionalism (wheee! for short). The equation is trivial to apply and will almost certainly guarantee my happiness with the shopping experience.
Wardley Happiness Equation of Extreme Exceptionalism
If I'm spending over £70 with you, then you can assume that this is part of a weekly shop. Hence when you come to tot up the final prices, simply calculate the delivery time plus 7 days and then for each item discount 15% for every day which the best before date is less than this.
Please don't ask me to check the dates or tell me that lettuce has a shelf life of three days - it's nonsense. There is nothing that annoys me more about online shopping than a week's worthy of grocery turning up with some best before dates of tomorrow.