Rest assured, re-sending proposals is absolutely free.
Your allowance is only deducted the first time you send a proposal, so if there is something that wasn't quite right the first time and needs changing, you can edit and resend a proposal at no extra cost.
To do this, locate the proposal you need to change under the Proposals tab. From here you can click Edit Proposal, and make any necessary adjustments. After that you can use the Re-send Proposal icon to send it off again - you won't be charged for doing so.