1. The transaction fee is based on sales price. You may of course reduce your commission to whatever amount you choose, but remember the transaction fee is based on sales price, not what you have reduced your commission to.
2. Agents may have one exemption in transaction fees for the purchase, or sale, of their primary residence. If you are selling your primary residence, and purchasing a primary residence, you would pick the most expensive one, and choose that one for your primary exemption. 🙂 Personal Investment properties are not exempt from transaction fees.
3. If you assist a family member or a friend (even with a FSBO) , and reduce your commission, the transaction fee is still based on the sales price. Please make sure that you factor your transaction fee into whatever commission you end up negotiating, so that your fee, and the transaction fee, are both covered. 
4. To deliberately reduce the sales price to obtain a lesser transaction fee, rather than the actual sales price, is unethical and is committing fraud.

Thanks!