You are buying as recommended by the manufacturer. At 5.7V nothing should happen, if the amperage is too low you may have an unstable circuit, although I doubt it. I bet there is some voltage stabilization in the Raspberry. If the plug does not fit, you earn a new one, or cut the tip from a micro-USB socket. Before connecting, you check several times if you have made a mistake. The cables are insulated with heat-shrinkable sleeves, to avoid the "cocoon" you move them by, say, a centimeter. You do not cut the plugs at the very "butt".
