Yes, this is a tough challenge because you are basically an exchanger and you are exposed to "exchange rate risk" until you sell your BTC. Ideally, you would sell the BTC into the exchange immediately at the time of receipt so you can lock in your spread (if charging a markup). Using the 24 hour average is bad because, as you stated, people are more inclined to spend with you vs. cashing out when the exchange rate you are offering is better for them than the current market rate.

It is not a big deal to sit on BTC if BTC is appreciating against the currency you are exchanging for but when the value of BTC drops like this last week your losses can pile up quickly.

Solutions:

1. Have a slush fund that will help you weather the dips that can hopefully be made up for if and when the BTC value rises again. This is similar to professional gamblers who have a large enough "bank" to weather long losing streaks until their luck changes back into their favor (perhaps you can take on a partner who is bullish on BTC and is willing to finance you?)

2. Charge a mark-up to give you an added cushion to help weather the down periods. After all, you are providing a service and people will be willing to pay for this service.

3. Figure out how to automate the selling of your BTC into the exchange as soon as you receive it (this means you can't use the 24 hour average) thus locking in your sales price. This is a hedge that bullion dealers like myself use when selling and buying bullion although I don't know of any way to automate this and you still have "exchange rate risk" until the trade is completed.

4. A combination of all of the above.

In summary you are basically a currency trader/speculator. The problem is that sellers of BTC "put" it to you (sell to you) when the BTC buying power is strong thus you are always buying BTC at the top. When the buying power of BTC drops your sales drop off so you don't get to accumulate much BTC at the lower prices. The way your business is structured now, you are betting that BTC will continue to appreciate against other currencies. When it doesn't you suffer losses.

All in all this is a tough situation to be in. You are basically a proxy buyer and currency trader. About the only solution I can see is if you had big pockets and wanted to accumulate BTC regardless of the price swings. Having a steady income of cash from other sources that allowed you to continue buying BTC over time would make it ideal provided you had enough disposable income to cover the sales of your services (your buying of BTC).

I hope this helps!
Trader Steve