Olga K. It really depends which plan are you on.
Does your plan include API/HTTP option?
Keep in mind that you can optimize waterfalls as well.
Step #1: Go to the waterfall providers list
Step #2: Order providers from cheapest to most expensive (Icypeas #1 etc.)
Step #3: Remove providers that have exorbitant pricing
In this way you can still use waterfall while preserving tons of credits.
i suggest using trykit, it is really helpful before going to waterfall.
Other methord is look what is the company follow to write an email(work email) (Please use this as the last option)
Second Igor on this, I remove any at over 0.5 credits and I find ZeroBounce to be the best Validation Provider imo - if this doesn't find an email for you, your credits are reimbursed anyway so you're not wasting any if you don't get a hit back.