The spelling of /p/ is always either <p> or <pp>. The /ʃ/ sound is usually spelled <sh> but we also see <ch> in chic and <ti> in cautious.
The contrast is between a stop and a fricative, both voiceless but fairly well separated in the mouth. It is unlikely to be a problem.
Interesting pairs include
The mean density value is 2.9%. The list makes 149 semantic distinctions, a loading of 50%.
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Credit: John Higgins