is there anywhere detailing how much the ratio of redemption cash to actual cash must be plus all the shipping details and whatnot?
With the exception of Nationals (where Rob has to pay quite a bit extra to get cards air-freighted in), Redemption cash is counted as being real cash for product. So if you have $15 in Redemption cash you can buy 5 Priests packs or 3 TexP or a Tin (or whatever).
If you pay for product with Redemption cash, then you need to pay actual cash for shipping and handling. This costs $5 for every $25 (or portion thereof) of product ordered.
All this is to the best of my memory. I bought two or three display boxes of Disciples with Redemption Cash last year.