I think "Many thanks!" should be accepted!
"Many thanks" would be the informal 多谢（了）！.
'Many thanks' sounds much less formal to me than 'thank you very much'.
It is just as formal.
Thank you so much
Is this not accepted?
What's difference between 感谢 and 謝謝?
The former is to convey gratefulness as well as thanks, my own translation is 'something that touched you 感动了你 maybe that you are thankful for', while the latter is "thank you". I don't think there's such a distinction in English.
Extremely grateful - that's what it really means, but not accepted here.