Capital One Bank credit card is a fraudulent ripoff business that victimized many consumers. Richmond, Virginia

27% of credit card companies revenue from FEES. So they are very eager to fine you as much as they can. This is so profitable, that they do it despite being fined many times by different agencies.

What to do? I have three ideas:

a/ Leave them ASAP - unfortunately if you have outstanding balance it could be impossible
b/ Register for their on-line account service
(capitalone.com click on the upper left corner on: Manage Your Card Account - but read the terms)
Meanwhile Capital One will not charge for this, but your bank might. (Usually around $5-10/month. It is cheaper than the $29 late fee etc.)
c/The easiest, cheapest way: send your letter as Certified Mail. It is only $2.30, but the USPS keeps record (for 2 years) of the mailing and delivery date, and you can get a receipt to prove when you mailed it. The destination post office can prove the date of delivery.

Usually banks do not play (as much) with people who use Certified Mail...