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Assume A has a partner named B. Last week both made an appointment to have dinner (dating). For a certain reason, A cannot come for the appointment and calls B by phone saying
・・・キャンセルさせてください。 Please make me cancel ...
A wants to cancel the appointment, but why does A ask B to make A cancel it as if the source of problem is on B's side? I think A should say
・・・キャンセルしてください。 Please cancel ...
It works like "Please let me", so
キャンセルさせてください = Please let me cancel [our appointment]
(It wouldn't be interpreted as "Please make me cancel [our appointment]".)
You could also say
キャンセルしてください = Please cancel [our appointment]
but it is a lot more direct (and thus less polite).
I want to expand @Earthliŋ's above very good answer, but it will not fit in a comment so I place it here.
使役形 itself is not 謙譲語 or 丁寧語, but it softens a request and is often used together with 謙譲語 or 丁寧語.
謙譲語
If you are asking politely to be allowed to use a toilet you could say
トイレを使わせて頂けませんか？（使役形＋謙譲語）
However, without 使役形, 謙譲語 would be unacceptable
NG トイレを使って頂けませんか？（終止形＋謙譲語）
丁寧語
A more direct way to ask to use the toilet by formulating with 使役形
トイレを使わせて下さい。 （使役形＋丁寧語）
However, with 丁寧語, 使役形 is not required as in:
トイレを使うことができますか？ （丁寧語）
トイレを使っても良いですか？ （丁寧語）
There are 使役形＋丁寧語 formulations which are not allowed as in:
NG トイレを使わせますか？ （使役形＋丁寧語）
NG トイレを使わせることはできますか？ （使役形＋丁寧語）
NG トイレを使わせてもいいですか？ （使役形＋丁寧語）
未然形＋丁寧語ではなく「使役形+丁寧語」とか「(動詞の)未然形+使役の助動詞+丁寧の助動詞」とかになるのでは？