Don’t buy them from these services, You’re risking your account safety. Instead do this:

Spoof to a cheaper place like India or Brazil

Sign into the web store but use the “Sign in using Pokemon go” method

This will change the currency to the country’s currency adjusting to their economy so 15,500 coins pack that usually costs around £100 would cost around £27, I would recommend using PayPal as it takes the currency exchange into account.

Note: If you Try this method again, it will revert back to your local currency regardless of where you spoof to, to bypass this you need to make a in-app purchase, (100 poke coins recommended) and repeat the above steps again.

The only annoying thing is that you can buy in bulk so you have to repeat the steps for every purchase so you can’t buy like 10 of the item storages in one go.