I do freelancing on the side and would use Paypal with smaller clients, but now I'm getting larger clients and wanted to make it more professional. I started using a free app a while ago and then realized I can only have 6 free invoices which just ran out. It also doesn't have an option for reciepts, which I just got asked for for the first time.
definitely recommend waveapps.com in that case. Free plan and no limit on number of customers or invoices, does receipts and links to your bank account, pretty much everything you need as a sole trader it seems!