Make sure that your shelf us close fitting and has a lip around the edges (it only needs to be 5 to 10mm high). Screw a
chain plate staple at either side of the shelf:
View attachment 250524
and use one (or more) of
these :
View attachment 250525
to keep the boxes in place as you go hare-arsing round country lanes
The sizes of screws you get depends on the jobs you are doing. At one time I bought a big Spax selection which was about £50 at the time and just added to it over time (mostly Spax, Reisser and ForgeFast screws). On the one hand for structural stuff you'll find sizes 5 x 50/70/100 and 6 x 70/100 very handy, whilst door ironmongery requires 3 x 12/16/20/25 and 3.5 x 12/16/20/25s
in BZP finish. For kitchen work I have a selection of Hospa screws, small angle brackets, Euro screws, shelf pins, door knob screws, csbinet joiners, etc. As I said - different kits for different tasks, and everyone has different screw needs