If I understand your question correctly you are pre-paying for postage. There are 2 options here:
1) Create a QB account called pre-paid franking. As you add funds to the actual account record the check in QB from your bank account with the expense account set as this new pre-paid franking […]

Resolution for 'accounting specific issue: 'I have cash in a prepaid franking account and still have over £200 left in it, how do I mark this in on quickbooks?' available: Yes.

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