A concrete paver border works okay, but is quite a bit of work to install. I put mine in to save myself some effort with the string trimmer, and be able to mow up to (and over) the edge of the lawn: The look doesn't appeal to everyone, but it's sturdy, durable and generally attractive, to my eye anyway. More pictures and an install guide here: http://havingfunalongtheway.com/building-a-mow-strip-for-your-lawn-part-1/

It's true, you can install a brick/paver border without a retainer, but the likelihood of bricks getting knocked out of place over time is pretty good. The retainer on the bark side is not as strong as that on the soil side. Compression of the base, the wedge of pressed soil, gravity and weight of bark mulch all contribute collectively to make it good enough. It's not going to stand up to a vehicle driving over the edge, but neither is that likely to happen. If you delete the retainer on the lawn side, again, this will not hold up over as long a time. But then, again, it may be good enough. You must be the judge about how long you expect the work to endure. To install retainer at lawn side, you must wound the lawn a little. It heals. If I was doing this for myself and wanted it to last several years, I would go old school (like I usually do) and install a concrete retainer at both sides. It takes a bit of a knack to learn to do this, but I wouldn't call it difficult. I put the concrete in the little "trench" with a garden trowell and shape it with a standard masonry trowel. The concrete mix must be a bit stiff. I get 9 linear feet of edge (one side) out of a 50 lb. bag of concrete mix. A bag of mix is a little less than $3 (here.) Just FYI.

I'm still not clear as to the sun's angle. Does the sidewalk go north/south, east/west, or some other direction? I have a bumpy lawn at my new house. I have bought a couple bags of generic topsoil and a bag of generic compost to fill the holes. My target mix is 10% compost mixed in with the topsoil. Some of the holes are surprisingly deep. I have 5 gallons (roughly 20 liters) of soil in one hole and it could another 5 to feather it out and match the surrounding profile. I have many holes to fill, but I started with the one that almost turned over the riding mower first. My yard was 95% weeds when I bought last summer. With our rainy winter and spring the real grass is spreading quite well. It might be down to 90% weeds already!! So filling the holes is easy when you don't care about smothering the plants living there. If I had an established lawn I would take it much more slowly and only add 1/2 inch (2-3 cm) at a time.

Strangely, it often is. I think China must have state subsidies on shipping because it's often free or very cheap. That particular business has a minimum order of 100 pieces, I think.