The room and board invoice is less than the listed Cost Of Attendance. Up to what amount (from these two) can I use my 529 funds for?

[sachinappa]

The college website lists the different Costs of Attendance for the different scenarios (Lives w/ Parents, Lives On Campus, Students w/ Dependents). My son stays on campus and is in his first year. The college website says that the cost for Housing and Food is $18,405. The meal plan that my son chose brings the cost for Housing and Food to $12395. This is the amount on the invoice. He will also be buying food outside of his meal plan both on campus and off campus. Let's say through these additional purchases, he spends another $7000 a year on food.

My question is - What amount can I withdraw from my son's 529 account as eligible expenses: $19395 ($12395 + $7000), $18405, or $12395?
Assume I will have receipts for the additional $7000 expense.

[AmeliesUncle]

$18,405

You can use allowable expenses ($19,395) UP TO the greater of (a) Cost of Attendance ($18,405) or the actual bill from the school ($12,395).

[sachinappa]

Thank you for your advice. 
I am assuming that it is best to keep the receipts for each of those expenses, adding up to $18,405, to avoid any IRS follow-ups. Given that many of these receipts will be in the $8 - $15 range, there will be 100s of such receipts to maintain. 🙁
Is it necessary to maintain all of these receipts to reimburse the full amount of $18,405?

Second question:
- given that this year my son will only incur expenses for 3.5 months, does the COA need to get pro-rated?

[AmeliesUncle]

Yes, if the IRS were to audit you, you would need all of those receipts.

Yes, it would be prorated, but I'm not exactly sure how it would be prorated because I'm not sure if the $18,405 would be for 12 months, or only the 9-ish months of school.  The school may be able to tell you the Cost of Attendance for those months.