Can you recommend a simple online payment system?

Question

Payment systems

Good day. Please tell me a simple online payment system. I need the following requirements:
- Acceptance of QIWI and Yandex.money, Webmoney is desirable, but not required.
- Possibility of payment for an arbitrary amount (as will be transferred from the site), and, accordingly, notification of successful payment to the site
- And most importantly, without problems with signing contracts or a bunch of requirements for the site. The turnover is planned to be very small, 5-7 thousand per month, maximum - 10, but not in the near future.
Thank you.

Answer 1

robokassa.ru
interkassa.com
payanyway.ru
pay2pay.com
onpay.ru
waytopay.org
paymaster.ru
payonline.ru
free-kassa.ru
paysto.com
walletone.com
xsolla.com
_ _
_ I hope my answer was more extended than the link.
Pay special attention to the first three of the list.

Answer 2

Yandex opened a new service, like ok.