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The repossession letter process set forth on the cited web site is a proprietary process similar to their so-called HIPAA process, and thus is not something that can be discussed in procedural detail on this forum.
Suffice it to say that use of FCRA 609 as requiring a furnisher to provide information within a period set by a consumer is not something that is supported by the statute per se or by any precedential case law.
I would advise caution in making demands that are not supported by law.
You can always, without making demands or your own interpretation of law, simply send an offer to pay what you feel is the obligated debt.
Debt collectors are explicitly permitted under FDCPA 808(1) to add amounts to the asserted debt that are in addition to the principal amount of the debt, such as interest or fees, provided they are authorized in the contract that created the debt or otherwise permitted by law, so it is not per se improper for the asserted debt to increase above the principal amount. Consumers are permitted to make an offer to settle for less than the asserted debt, so rather than immediately follow some confrontational process, it might be best to first seek their good-will by making a settlement offer.