With Simple Delivery listings the postage is fixed – you cannot change it once an item has sold. (You can only cancel the entire order.) You can opt not to use the Simple Delivery postage label, but then you would be paying the postage cost yourself. So that’s really not a viable solution.
Anything under £5 that you can opt out of Simple Delivery then do. Paying £1.15 extra in postage (just for tracking) is silly, as that extra ~40% means unless 1 in 3 are getting lost, then financially it isn’t worth it. As the real number should be 1/50 worse case, then it really isn’t worth it.
But do note that there are more issues these days with delays, and impatient buyers. So you may have to end up refunding some items that do eventually get delivered. As mentioned by @dancewithadingo, Simple Delivery can protect you from those issues.
You also need to be careful that any DVDs don’t exceed 100g. It’s been a long time since I sold any, but I vaguely remember some just going over weight (I think thick booklets and double discs were the prime culprits). But if you are checking the weight before posting, then paying an extra 25p out of your own pocket isn’t much of an issue. But if you’re not checking (properly), then a surcharge to your buyer from Royal Mail can be bad for your reputation.
So I would suggest…
If you don’t really care about the money, then go with Simple Delivery. The protections offered by eBay means it’s relatively safe to sell that way.
If you want DVDs to have more chance of selling, or be able to charge up to 50% extra (as the total with postage will be lower), then go with standard second class large letter service.
