Enter a number between 0 and 99,999,999

That's not a y2k issue, it's a hard stop coded into the program.

The only workarounds I know of is to break the transaction into multiple transactions or increase the cost by some power of 10 and reduce the quantity accordingly.

Would sure like to know if someone at Intel, AMD and/or Microsoft and all the other UNIX providers have figured out a way to support

decimal instructions with more than 15 digits length

C# has a decimal type.

The decimal type is appropriate when the required degree of precision is determined by the number of digits to the right of the decimal point. Such numbers are commonly used in financial applications, for currency amounts (for example, $1.00), interest rates (for example, 2.625%), and so forth. Even numbers that are precise to only one decimal digit are handled more accurately by the decimal type: 0.1, for example, can be exactly represented by a decimal instance, while there's no double or float instance that exactly represents 0.1. Because of this difference in numeric types, unexpected rounding errors can occur in arithmetic calculations when you use double or float for decimal data. You can use double instead of decimal when optimizing performance is more important than ensuring accuracy. However, any difference in performance would go unnoticed by all but the most calculation-intensive applications. Another possible reason to avoid decimal is to minimize storage requirements. For example, ML.NET uses float because the difference between 4 bytes and 16 bytes adds up for very large data sets.