I don't believe they will accept a credit card or check so have either USD or Bz with you to pay the duties. Make sure to bring some small bills too because at times I've had trouble and delay because they couldn't make change. The whole process is very easy. I have never volunteered the receipts and have only needed them on electronic items because they had no idea how to estimate value on such items.
It varies depending on who the individual custom officer is but overall they are fair and OK to deal with as long as you are somewhat reasonable on your reporting. Duties vary on classification of goods brought in. Keep smiling and be respectful. One time I had the customs officer lower my estimated value.