Lee Jeans Misleading Discount Complaint

I found a shirt I liked on the https://www.lee.com.tr/ website, which advertised a "25% off in cart" discount. I eagerly awaited this discount at checkout, but thankfully, I was cautious with my purchase. After multiple attempts, I noticed the discount wasn't being applied.
I reported this issue to the website's customer service. Their initial response was to request a photo, but the discounts kept changing. At one point, there was a 20% discount on two items, but when I tried to apply it, it didn't work either. Later, I noticed the "25% off in cart" offer had returned, but it still wasn't reflected in my cart. I took a screenshot and sent it to them.
In response, they sent me a 20% discount coupon. Pleased with this, I purchased the shirt. However, half an hour later, I checked the site again and saw that the "25% off in cart" discount had returned and was actually being applied correctly this time. They had apparently fixed their technical issues, and I realized I had overpaid by 5%.
When I reported this, their response was unsympathetic. They explained that discounts change moment by moment and that I should constantly monitor them. The issue isn't about overpaying by 5%, but rather how a reputable company can behave like a street vendor, sending discount coupons to compensate for technical problems while actually charging 5% more. I feel misled and disappointed by this experience.