As per the PA rev. code
Tax is to be collected on the full purchase price without any deduction on account of the cost of labor, shipping, handling, delivery, or installation. Separately stated charges for returnable containers are not subject to tax.
Since late fees are not part of purchase price, you do not collect sales tax on late payment charges.
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Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases. Nothing contained in this reply was intended or written to be used, can be used by any taxpayer, or may be relied upon or used by any taxpayer for the purposes of avoiding penalties that may be imposed on the taxpayer under the Internal Revenue Code of 1986, as amended.