Reg. Section 1.461-1(a)(1) provides the following:
If an expenditure results in the creation of an asset having a useful life which extends substantially beyond the close of the taxable year, such an expenditure may not be deductible, or may be deductible only in part, for the taxable year in which made.
https://www.law.cornell.edu/cfr/text/26/1.461-1
If you buy 10+ months of AWS credits in December and have a Jan-Dec fiscal year, I'd argue that you bought "an asset having a useful life which extends substantially beyond the close of your taxable year"
