What app is charging me $5.40 a month? I have no clue what this charge is for

Click on the link below, you can

• View your purchase history

• Cancel a subscription

• Request a refund

• Verify unfamiliar charges

• Add or remove a payment method

Subscriptions and Billing - Official Apple Support

See this article.

View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support

Well, your original post said: “What app charges …”. Then your second post said “There is no app”. Clearly these cannot both be true.

So it’s not at all clear where you might be seeing this charge. If it’s not in your purchase history, I will guess perhaps it’s here.

If you see 'apple.com/bill' on your billing statement - Apple Support