WASHLET+ sets
These are complete sets — the toilet is designed around the seat, so the power cord and water line run behind the skirted base rather than across the floor.
That is not something you can achieve by adding a seat to an ordinary toilet, which is why the sets exist as their own product.
Why instant warm water matters
Cheaper seats heat water in a small tank. It lasts around 40 seconds and then runs cold until it reheats, which is noticeable if two people use the bathroom in a row.
The S7 heats on demand, so it never runs out. It is the single feature most worth paying up for.
S7 or S7A?
The S7A adds auto flush. If that is not important to you, the S7 gives you the same wash for less.