For the credit card payments on a tablet/phone, I think you really only have an choice of Square. Their fees are in line with other card processors. I haven't used their services as a merchant, but I've purchased things from merchants who use them, and it's painless from that perspective. I was looking into using them for my wife's theatre, but we currently get POS card payment processing done for free via one of the city departments, so decided to forgo the extra $30/month data plan and processing fees.

If you haven't found it already, it seems that ServiceBridge does all the rest.

As for digital signatures, she's probably fine without a paper trail (although she could have one printed at the central office) -- the last few times I've had repairmen out for something, I signed on their tablets. One of those was for Sears, so if Sears is doing it that way...