The Marines, and perhaps the Navy (both are under the Navy's pay system as I recall) don't get paid while in basic. You are not required to meet this request, but in most circumstances I don't believe you'd be criticized either. Having a signed request for an extension is a bank rule in most cases. There could be a state requirement, I wouldn't know. Rather than go through the pain of sending a request to be signed and returned, I'd respond (if you do this) that you have met the request. Tell them how much the interest is and when it is due. Hopefully you can delay that until the soldiers payday. Tell them when their new maturity is and what they need to start paying, and when.
You're other option is to SCRA the rate to 6% and push them for payments. If the soldier doesn't have a relative to pay for them, you won't have anything but a past due account. There is nothing they can do.
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AndyZ CRCM
My opinions are not necessarily my employers.
R+R-R=R+R
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell