1. There cannot be a sole proprietorship company. You can have a sole proprietorship firm.
2. A sole proprietorship requires no registration, but it is advisable to register because banks normal insist on registration for opening bank accounts in the name of the firm.
3. For online payment, the payment gateway should be the one approved by RBI,
4. IT return will have to be filed by you if your income, after applying all deductions, exceeds the exempted income.