The question is, can we use GPC to purchase monthly propane gas (refueling) for space heaters?
The Government Purchase Card (GPC) is, by definition, a Simplified Acquisition Procedure (SAP) procurement instrument; its use is governed by FAR Part 13, as supplemented by Agency regulation/procedures. In reviewing the FAR and its supplements, there is no express prohibition on the purchase of propane fuel using a GPC that I can find. For your question, the biggest consideration is the prohibition on split purchases at FAR 13.003(c)(2), which is reiterated at AFARS 5113.270-90(b) with specificity to the GPC. Your customer knows their requirement is recurring, so you need to look at it in aggregate for the FY, not monthly, as the requirement is continuous and predictable with a reasonable degree of confidence. When looking at the requirement from you customer, will the propane needs for the year exceed the micro-purchase threshold (MPT)? I note you question mentions your end user is deployed to remote locations - if a declared contingency, the MPT would be increased; if not a declared contingency, the MPT remains unchanged.
If based on the above, the GPC is not a viable purchasing option (requirement exceeds MPT), GPC may still be a valid payment option. A technique commonly employed in scenarios similar to yours, is to establish a BPA, which creates an ordering vehicle; order placement (BPA Call) can then be delegated to your customer if the KO is agreeable. Your customer may place orders up to the threshold the KO authorizes IAW regulation and make payment using the GPC. This would require some additional effort on both the contracting office and customer to establish and administer a BPA, but may be less administratively burdensome than separate contract actions each month. Additionally, by utilizing a BPA, you provide for price list updates to account for the volatile nature of fuel pricing. Another contract vehicle possibility, would be a requirements or IDIQ type contract, leveraging an Economic Price Adjustment (EPA). This would provide flexibility in ordering, to include possible delegation to customer, again, subject to KO discretion; the EPA would reduce pricing risk to both Government and Contractor associated with the volatility of propane pricing.