Donating between $250-$500 multiple times per year to same charity requirements

I see that for noncash donations between $250 to $500 in value, a receipt or letter that includes the charity's name, address, date, donation location, and description of items donated is needed. I donate frequently and just drop off donations and they give me a receipt (many times it's blank and I fill in the date). I then attach a list of what I donated to that receipt.   Is this sufficient?  Or does the charity need to actually write a letter confirming what was received?  Unless donating something much more valuable, charities don't do this.  I do fill out the 8283 form and am donating to the same charity.
 
Is this requirement for each individual donation and not the total per year? 
 
If a letter from the charity is required, is it ok to donate many more times so the total each time falls below $250? If I donate $400 worth of non cash items 10 different times for a total of $4000, could I instead donate $200 worth of items 20 times a year?