In general, yes.
Payments to RateX is considered online spending and users will benefit from online spending rebates. This is assuming there are no restrictions on the online merchant you have to pay to.
There might be cases where online shopping rebates are restricting to certain goods category e.g. clothing, groceries. In such cases, we have to look in Merchant Category Code (MCC). The MCC does define cashback for the card issuer. Things like food will have an MCC which means the issuing bank can calculate the cashback per transaction.
RateX MCC is 5399 (Miscellaneous General Merchandise).
Note: If the cashback is specifically tied to Amazon purchases, then there are no credit card rewards/cashback.