It depends on the plastic. The petroleum distillates like lighter fluid, Goof-Off, and non-chlorinated brake parts cleaner, or even WD-40 are not going to react with polyethylene and polypropylene, but they can potentially dissolve ABS and PVC types as well as acetate and acrylics.
For sticker removal, I recommend this tool from Lisle:
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Rather than starting with the metal razor blades it comes with, use these ABS blades from Foshio instead:
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If the plastic is compatible, Goof-Off or Acetone are good adhesive removers. Note that the orange ABS blade will suffer, but they're disposable. They are better than credit cards because their edge is sharper. Credit cards are PVC and will also suffer from use with solvents. Step-up to the metal blades for stubborn jobs or if the surface is hard enough like glass or metal with a flat surface that won't be scratched. If the plastic is not compatible with a solvent, I recommend Goo-Gone.
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Depending on the tolerance of the material for heat, a heat-gun can also help. Sometimes just mild heat at hair-dryer levels will soften the adhesive and help lift it off. If the paper sticker is on a heat tolerant material, it might be possible to burn it off with the heat gun or a propane torch.