It's possible to both up sell and have a good UX. Provided the upsell UI is sensibly restricted, unobtrusive and actually related.
Sometimes up selling can actually helpful. I brought a camera recently because I dropped my last one and smashed the screen. After adding it the basket the site listed a few related accessories which included a leather case. Following my recent breakage it's exactly what I needed, but for some reason I had totally forgotten. As it happens I searched a bit a got a different case, but it was their upsell prompt that reminded me I needed one.
Sometimes up selling can actually helpful. I brought a camera recently because I dropped my last one and smashed the screen. After adding it the basket the site listed a few related accessories which included a leather case. Following my recent breakage it's exactly what I needed, but for some reason I had totally forgotten. As it happens I searched a bit a got a different case, but it was their upsell prompt that reminded me I needed one.