The data is the same.
8.75"x11.25" at 300dpi is the exact same size as 36.47"x46.89" at 72dpi - the only difference is how the data is displayed, but it's the same data.
If you increase the PPI in the 72ppi file without resampling pixels you end up with an 8.75x11.25" file. There is no difference.
Note in the animation above the
Dimensions do not change - they are still 2626px x 3376px. All that is being altered is the density of the pixels for output, resulting in a reduction of physical dimensions.
It's the exact same data.
Any print provider that is unaware of this would certainly not be worth my money and I'd have some difficulty trusting that they know what they are doing.
Simply because some app somewhere is assuming the PPI is 72 that doesn't mean the data is insufficient for print quality output.
To put it rather bluntly, your print provider doesn't know what they are doing. Find another one.