"Players can store up to 20 tradeable bonds and 20 untradeable bonds in their bond pouch"
I believe that you simply can not put more than 20 of these bonds in your pouch. Thus forcing you to keep it in your inventory/exchange a bond for days.
This equates to about a full year of membership. This limit might have something to do with accounting, since any debt that lasts over 1 year will be a long term debt, rather than a short term debt (it's presented in different places). Or it might just be that 1 year membership is the longest membership term they offer through the regular way.Vintage player of runescape since 2003.